Relaxation of interstitials in spherical colloidal crystals

Spherical colloidal crystals (CCs) self-assemble on the interface between two liquids. These 2D structures unconventionally combine local hexagonal order and spherical geometry. Nowadays CCs are actively studied by altering their structures. However, the statistical analysis of such experiments results is limited by uniqueness of self-assembled structures and their short lifetime. Here we perform numerical experiments to investigate pathways of CC structure relaxation after the intrusion of interstitial. The process is simulated in the frames of overdamped molecular dynamics method. The relaxation occurs due to interaction with extended topological defects (ETDs) mandatory induced in spherical CCs by their intrinsic Gaussian curvature. Types of relaxation pathways are classified and their probabilities are estimated in the low-temperature region. To analyze the structural changes during the relaxation we use a parent phase approach allowing us to describe the global organization of spherical order. This organization is preserved by only the most typical relaxation pathway resulting in filling one of vacancies integrated inside the ETD areas. In contrast with this pathway the other ones shift the ETDs centers and can strongly reconstruct the internal structure of ETDs. Temperature dependence of the relaxation processes and the mechanism of dislocation unbinding are discussed. Common peculiarities in relaxation of spherical structures and particular fragments of planar hexagonal lattice are found.     

Observation of edge dislocations in helical smectic liquid crystals

We report the observation of elementary edge dislocations in smectic liquid crystals possessing helical structure. The dislocations were observed in the entire temperature range of helical phases, including ferroelectric, ferrielectric and antiferroelectric phases. The mechanism for visualizing the dislocations is based on the phenomenon of selective reflection of circularly polarised light. The performed observations of dislocations deliver not only information on the mechanisms of defect creation in various chiral smectic phases, but also on the structure and properties of the investigated smectics, often inaccessible using standard methods.     

Influence of limited defects in crystals on the Kikuchi lines

The electron-diffraction patterns from silicon with curved Kikuchi lines were obtained. It is revealed that curving of Kikuchi lines takes place simultaneously with the shift of point reflections from their normal positions. It is found that curving of Kikuchi lines is caused by limited defects in silicon crystals.     

Solitons and defects in nematic liquid crystals under a simple shear flow and in a static external magnetic field

Under a simple shear flow and in a static external magnetic field, the production of defects in the director-aligning regime of nematic liquid crystals has been investigated in terms of the Leslie-Ericksen theory. The equation of motion of the nematic director, which conforms to the driven over-damped sine-Gordon equation, has a soliton solution of the amplitude w. We show that the stationary state with the director uniformly oriented at a Leslie angle is only a metastable state and the potential, which governs the motion of the director, has a nmnber of stable stationary states. For a strong magnetic field, the higher energy barrier between the stable and unstable states leads the director to be locked along the magnetic field direction. However, at the appropriate shear rate and magnetic field the defects, which appear as a stable solitary solution, can be nucleated from a uniformly aligned nematic liquid crystal. We have calculated the stationary travelling velocity of the solitary waves and the distance between a pair of defects.     

Hydrothermal etching as a method of quartz quality control of density and distribution of dislocations

Abstract

In this paper the effective non-destructive method of hydrothermal etching of crystals is considered to carry out estimation of density and place of dislocations in large scaled crystals of synthetic quartz. In this process of etching there are specific etch figures which appear as faceted holes of a millimetre up to several millimetres in size on all quartz facets. The size of holes depends on concentration of solution, duration of the process of etching and degree of solution saturation by silicon dioxide. The configuration of the holes on the facets of different indexes is various. Pits of symmetry, having facets, correspond to each facet there. Zcrystals are basic in quartz production. In sectional operation the expedients of defection of dislocations in pits generated on a facet are considered (0001).     

On the deflection of light by topological defects in nematic liquid crystals

The influence of controllable parameters like temperature and wavelength on the trajectories of light in a nematic liquid crystal with topological defects is studied through a geometric model. The model incorporates phenomenological details as how the refractive indices depend on such parameters. The deflection of light by the topological defect is then shown to be greater at lower temperatures and shorter wavelengths.     

Topological defects in the left-right symmetric model and their relevance to cosmology

It is shown that the minimal left-right symmetric model admits cosmic string and domain wall solutions.     

KDP晶体本征中性点缺陷的第一性研究

 用第一性原理研究了KH2PO4(KDP)晶体中性本征点缺陷的形成能并计算了常温下点缺陷的浓度。计算得到中性填隙氢原子的形成能为2.05 eV,进而得到298 K下的浓度约为1.21×10^-17 mol/L。由于填隙氢原子在带隙中形成缺陷能级,并使能隙降低了2.6 eV, 因此消除填隙氢原子有利于提高晶体在355 nm附近的激光损伤阈值。计算得到的氧间隙、氧空位、钾空位和氢空位的形成能分别为0.60、5.25、6.50 和6.58 eV,常温下它们在晶体中也以较高的浓度存在。钾空位使晶胞体积增大约3.2%,并可能提高晶体电导率,从而降低光损伤阈值。P取代K的反位结构缺陷形成能尽管较低(4.1 eV), 但由于晶体生长溶液中P是以PO4四面体的形式存在,故此点缺陷的存在几率很小。     

Electron on a cylinder with topological defects in a homogeneous magnetic field

In this work we study the effects of the geometry and topology of a cylinder on the energy levels of an electron moving in a homogeneous magnetic field. We consider the existence of topological defects as a screw dislocation and a disclination. When we take the region of movement as the full cylindrical surface, we find that, by increasing the strength of the screw dislocation, the dispersion on the electronic energy levels is affected and monotonically increasing. For an electron moving in an almost flat region we show that the dispersion on the Landau levels decrease monotonically as we increase the strength of the screw dislocation. The lowest Landau level can reach a zero value, leaving the energy of the system solely given by the geometry of the cylinder, which does not depend on the magnetic field. In both situations, as we change the deficit angle of the disclination, we observe that the energy levels are shifted and the magnitude of such shift depends on the magnetic field. The Landau levels for a flat sample are recovered in the limit of an infinite cylinder radius.     

Radiation-induced modifications of point defects in quartz crystals and their application in radiation dosimetry

Different types of impurity-related point defects in crystalline quartz are known to exhibit various spectroscopic signals. When exposed to ionizing radiations, these defect centers get modified and new species of point defects are formed. This paper presents a study and discussion of radiation-induced modification of point defects in natural as well as cultured quartz, with an emphasis to use this material for radiation dosimetry up to a range of a few Mrad.     

NMR probe of bulk electronic structures in H+ beam irradiated Bi2Te3 topological insulator

We report a nuclear magnetic resonance (NMR) study on H⁺ beam irradiated Bi₂Te₃ powdered single crystals. In this work, we demonstrate that the beam creates defects within its penetration range giving rise to delocalized charge carriers, thereby making further ¹²⁵Te NMR Knight shift and line broadening. Upon increasing temperature, the NMR line narrowing manifests the activated motions of thermally excited charge carriers in the irradiated sample. In contrast, it reveals that in the unirradiated sample the free-charge carriers at the Fermi level dominantly contribute to the Knight shift. Our results show that the orbital contribution to the Knight shift in the bulk state of Bi₂Te₃ becomes predominant in the system with the higher density of defects, as evidenced by modified electronic structures induced by the beam irradiation.     

Scattering of charge carriers in graphene induced by topological defects

We study the scattering of graphene quasiparticles by topological defects, represented by holes, pentagons and heptagons. For holes, we found that at low concentration they give a negligible contribution to the resistivity. Whenever pentagons or heptagons are introduced we realize that a fermionic current is scattered by defects.     

Electron paramagnetic resonance study of impurities and point defects in oxide crystals

In this topic review the results of the X-band electron paramagnetic resonance (EPR) measurements of Mn, Co, Cr, Fe ions in YAlO₃ (YAP) crystals and Fe ions in LiNbO₃ (LNO) crystals and of chromium doped Bi₁₂GeO₂₀ (BGO) and Ca₄GdO(BO₃)₃ single crystals, are presented. It is well known that the oxide crystals (for example:YAP, LNO, BGO) are one of the most widely used host materials for different optoelectronic applications. The nature of point defect of impurities and produced in the oxide crystal after irradiation by bismuth ions and after irradiation by the ²³⁵U ions with energy 9.47 MeV/u and fluency 5?×?10¹¹?cm^?1 is discussed. The latter is important for applications of these oxide crystal as laser materials.     

Thermodynamics of ion exchange defects in aluminosilicates

Summary The Debye-Huckel model for electrolytes is utilized to evaluate the density of defects originated by the exchange of Al and Si ions in aluminosilicate lattices.     

Elastic interaction of point defects in cubic and hexagonal crystals

The elastic interaction of two point defects in cubic and hexagonal structures has been considered. On the basis of the exact expression for the tensor Green’s function of the elastic field obtained by the Lifschitz–Rozentsveig for a hexagonal medium, an exact formula for the interaction energy of two point defects has been obtained. The solution is represented as a function of the angle of their relative position on the example of semiconductors such as III-nitrides and α-SiC. For the cubic medium, the solution is found on the basis of the Lifschitz–Rozentsveig Green’s tensors corrected by Ostapchuk, in the weak-anisotropy approximation. It is proven that the calculation of the interaction energy by the original Lifschitz–Rozentsveig Green’s tensor leads to the opposite sign of the energy. On the example of the silicon crystal, the approximate solution is compared with the numerical solution, which is represented as an approximation by a series of spherical harmonics. The range of applicability of the continual approach is estimated by the quantum mechanical calculation of the lattice Green’s function.     

二维磁振子晶体中线缺陷模的性质及其应用

在超原胞近似下, 利用平面波展开法数值计算了含线缺陷结构的二维磁振子晶体带结构及缺陷模的磁化强度场分布. 研究结果表明, 线缺陷结构的引入会在禁带中产生一个小范围的通带, 即产生线缺陷模. 该模式的存在可使自旋波沿着线缺陷结构的方向传播, 利用此性质含线缺陷结构的二维磁振子晶体材料可作为自旋波导波器件的制作材料. 关键词： 磁振子晶体 线缺陷 波导     

Tuning spin-filtering,rectifying, and negative differential resistance by hydrogenation on topological edge defects of zigzag silicene nanoribbons

By first-principles calculations, we propose three heterojunction nanodevices based on zigzag silicene nanoribbons with different edge-hydrogenated topological line defects. The devices all present excellent spin-filtering properties with 100% spin polarization as well as remarkable rectifying effect (with rectification ratio around 10²) and negative differential resistance behaviors. Our findings shed new light on the design of silicon-based nanodevices with intriguing spintronic applications.