Presence Of Screw Dislocations Research Articles

Electric quadrupole moment of a neutral non-relativistic particle in the presence of screw dislocation

In this contribution, we investigate the interaction between electric and magnetic fields with an electric quadrupole moment of a spinless particle moving in an elastic medium which has a topological defect (screw dislocation). By considering this interaction, the Schrödinger equation is exactly solved by using the analytical method. Thus, the eigenfunction and energy eigenvalues for two configurations are found. Meanwhile, by observing a shift in the angular momentum quantum number, the energy eigenvalues and the wave function of the system are modified, due to the screw dislocation in the medium.

Duffin–Kemmer–Petiau oscillator in the presence of a cosmic screw dislocation

We examine the behavior of spin-zero bosons in an elastic medium which possesses a screw dislocation, which is a type of topological defect. Therefore, we solve analytically the Duffin–Kemmer–Petiau (DKP) oscillator for bosons in the presence of a screw dislocation with two types of potential functions: Cornell and linear-plus-cubic potential functions. For each of these functions, we analyze the impact of screw dislocations by determining the wave functions and the energy eigenvalues with the help of the Nikiforov–Uvarov method and Heun function.

Using curves to make twists

Nanomaterials The growth of layered materials on flat substrates usually occurs in stacked layers, although defects or a lattice mismatch can induce strains that distort the shape of subsequent layers. However, these effects are usually small and can be uncontrolled. Zhao et al. now demonstrate the possibility of synthesizing multilayers of two-dimensional materials with certain twists between the layers induced by the presence of screw dislocations in combination with curved substrate surfaces. Different twist angles are achieved by varying the amount of nonplanarity and the character (conical or hyperbolic) of the surface. Science , this issue p. [442][1] [1]: /lookup/doi/10.1126/science.abc4284

Intensification of shock damage through heterogeneous phase transition and dislocation loop formation due to presence of pre-existing line defects in single crystal Cu

In general, shock wave deformation studies of perfect single crystals may cause disagreement with the experimental findings as the complete elimination of all defects in the metallic system is not possible in reality. Here, we have studied the influence of edge and screw dislocations on the intensification of damage produced during the propagation of shock at various velocities. Various analyses have been performed such as common neighbor analysis, atomic strain analysis, stress analysis, and kinetic energy mapping to investigate the underlying plastic deformation mechanisms. Results have revealed that the presence of edge dislocations has caused intensified damage through localized amorphization and phase transition. In comparison with the perfect crystal, the presence of pre-existing edge dislocations has incurred an additional damage of ∼17% to the specimen region. On the other hand, the presence of screw dislocations in the specimen causes damage through shear bands and dislocation loop formation, which is found to constitute greater than 80% of the specimen region.

Electronic Structure Calculations of Oxygen Atom Transport Energetics in the Presence of Screw Dislocations in Tungsten

Plastic flow in body-centered cubic (bcc) alloys is governed by the thermally-activated motion of screw dislocations in close-packed planes. In bcc interstitial solid solutions, solute diffusion can occur at very fast rates owing to low migration energies and solute concentrations. Under mechanical loading, solutes may move on the same or similar time scale as dislocations glide, even at low temperatures, potentially resulting in very rich co-evolution processes that may have important effects in the overall material response. It is therefore important to accurately quantify the coupling between interstitial impurities and dislocations, so that larger-scale models can correctly account for their interactions. In this paper, we use electronic structure calculations to obtain the energetics of oxygen diffusion under stress and its interaction energy with screw dislocation cores in bcc tungsten. We find that oxygen atoms preferentially migrate from tetrahedral to tetrahedral site with an energy of 0.2 eV. This energy couples only weakly to hydrostatic and deviatoric deformations, with activation volumes of less than 0.02 and 0.02 b 3 , respectively. The strongest effect is found for the inelastic interaction between O atoms and screw dislocation cores, which leads to attractive energies between 1.2 and 1.9 eV and sometimes triggers a transformation of the screw dislocation core from an easy core configuration to a hard core configuration.

On the role of dislocation characters influencing ratcheting deformation of austenitic stainless steel

Ratcheting (i.e., asymmetric stress-controlled fatigue) behavior of a non-conventional stainless steel X12CrMnNiN17-7-5 has been investigated with various combinations of stress amplitudes, mean stresses and number of cycles. XRD profile analysis following modified Williamson-Hall method has been carried out to estimate the dislocation characters and their respective densities in the ratcheted specimens. The precise role of these substructural features on the extent of strain produced during the course of ratcheting has been investigated. The increase in strain owing to ratcheting has been explained by the presence of type of dislocations and their respective densities in the constituent phases of the microstructure. The accumulation of cyclic plastic strain during ratcheting deformation is predominantly progressed by the presence of screw dislocations. The significantly increased fraction of edge dislocations in martensite phase resulted higher work hardening in the ratcheted specimens. Accordingly, the extent of strain produced is relatively less during subsequent ratcheting deformation.

Giant Optical Activity of Quantum Dots, Rods, and Disks with Screw Dislocations.

For centuries mankind has been modifying the optical properties of materials: first, by elaborating the geometry and composition of structures made of materials found in nature, later by structuring the existing materials at a scale smaller than the operating wavelength. Here we suggest an original approach to introduce optical activity in nanostructured materials, by theoretically demonstrating that conventional achiral semiconducting nanocrystals become optically active in the presence of screw dislocations, which can naturally develop during the nanocrystal growth. We show the new properties to emerge due to the dislocation-induced distortion of the crystal lattice and the associated alteration of the nanocrystal’s electronic subsystem, which essentially modifies its interaction with external optical fields. The g-factors of intraband transitions in our nanocrystals are found comparable with dissymmetry factors of chiral plasmonic complexes, and exceeding the typical g-factors of chiral molecules by a factor of 1000. Optically active semiconducting nanocrystals—with chiral properties controllable by the nanocrystal dimensions, morphology, composition and blending ratio—will greatly benefit chemistry, biology and medicine by advancing enantiomeric recognition, sensing and resolution of chiral molecules.

The effect of noise on the dirac phase of electron in the presence of screw dislocation

The effect of noise on the Dirac phase of electron in the presence of screw dislocation is studied. An uncorrelated noise, which coincides with the nature of thermal fluctuations, is adopted. Results indicate that the Dirac phase is robust against the existing noise in the system.

Growth of InAs nanostructures on InP using atomic-force nanolithography

Atomic-force nanolithography was used to control the nucleation sites of InAs nanostructures on InP substrates. Indentations with a wide range of dimensions were produced on InP. InAs nanostructures were selectively grown by metal organic vapor phase epitaxy. It is shown that the number of active nucleation sites depends on the normal force applied during nanoindentation. Crystalline defects introduced by nanoindentation are shown to be nucleation sites for these nanostructures. The presence of screw dislocations within the grown nanostructures further supports this observation.

Internal structure of clusters of partially coherent nanocrystallites in Cr–Al–N and Cr–Al–Si–N coatings

Nano-sized clusters consisting of strongly preferentially oriented, partially coherent nanocrystallites were observed in Cr–Al–N and Cr–Al–Si–N coatings deposited using cathodic arc evaporation. Microstructure analysis of the coatings, which was done using the combination of X-ray diffraction (XRD) and transmission electron microscopy with high resolution (HRTEM), revealed furthermore stress-free lattice parameters, size and local disorientation of crystallites within the nano-sized clusters in dependence on the aluminium and silicon contents, mean size of these clusters and the kind of structure defects. Within the face-centred cubic (fcc) Cr 1 − x − y Al x Si y N phase, the stress-free lattice parameter was described by the equation a = (0.41486 − 0.00827 · x + 0.034 · y) nm. The size of individual crystallites decreased from ∼ 11 nm in Cr 0.92Al 0.08N to ∼ 4 nm in Cr 0.24Al 0.65Si 0.10N. These nanocrystallites formed clusters with the mean size between 36 and 56 nm. The mutual disorientation of the partially coherent nanocrystallites forming the clusters increased with increasing aluminium and silicon contents from 0.5° to several degrees. The disorientation of neighbouring nanocrystallites was explained by the presence of screw dislocations and by presence of phase interfaces in coatings containing a single fcc phase and several phases, respectively.

Phase structure of helico-conical optical beams

We examine the phase structures of Helico-Conical (HC) Optical Beams produced via computer-generated holograms (CGH). The CGHs of these beams are made such that the reconstruction will have a phase which is a product of a helical and a conical phase. Interference-based phase measurement technique reveals very distinct differences between the phases of the two types of HC Beams (K=0 and K=1). Most obvious is the presence of screw dislocations at the center of the beam for K=1 while no screw dislocations are found at the center for K=0. We also compare the experimental interference patterns with numerical interference patterns calculated by superimposing a field containing vortices at different positions and a plane wave. A remarkable similarity is seen between the interferograms, which is a good indication of the phase of these beams. Contour plots are drawn to show the phases of these beams. We observe a string of vortices with equal charge in contrast to the assumption that a string of alternating charges should be seen.

Morphology and growth kinetics of 3He crystals below 1 mK

The shapes as well as the growth and melting properties of bcc-3He single crystals have been investigated with a low temperature Fabry-Perot interferometer. Eleven types of facets were clearly identified during slow crystal growth at the temperature of 0.55 mK, where the solid is in the antiferromagnetically ordered u2d2 phase. The growth rates of the individual facets have been measured and the results indicate significant growth anisotropy. The observed linear dependence of the growth velocity on the driving force shows that facets grow due to the presence of screw dislocations, while the step velocity is limited by the spin wave velocity due to the strong interaction of the moving step with magnons in the solid. The measured growth rates of the facets and the assumed growth mechanism gave us the unique opportunity to obtain the step free energies for ten different types of facets observed during a single growth sequence. The dependence of the free energy of the step on the step height is compared with predictions of the weak- and strong-coupling models. Our results suggest that 3He crystals have rather strong coupling of the liquid/solid interface to the crystal lattice and that the step-step interactions are of elastic origin.

Screw dislocation-induced enhancement of the c-axis critical current in anisotropic superconductors

Supercurrent in the weakly conducting c-axis direction of layered superconductors is studied. It is shown that in the presence of screw dislocations with Burgers vectors parallel to the c-axis, the critical current of small samples is enhanced with respect to the perfect crystal, each dislocation being able to carry a supercurrent I 0.

Parameterization and orbital angular momentum of anisotropic dislocations

Waves that have an isotropic intensity distribution about the propagation axis may carry well-defined, quantized orbital angular momentum. The angular momentum is nonzero in the presence of screw dislocations. It is shown that these dislocations can be represented by a complete orthogonal set of functions (such as the Gauss–Laguerre set, which is valid for the paraxial approximation). Representing anisotropic dislocations by a superposition of isotropic ones provides easy derivation of various quantities related to the angular momentum, such as its expectation value and the uncertainty. Concentrating on dislocations that are solutions of the Laplace equation, we propose two natural and convenient ways of parameterization that bring forth their isotropic components as well as the geometrical properties of their phase map.

{103}⟨331⟩ slip in MoSi2

Abstract {103}⟨331⟩ slip is found to be activated in MoSi2 single crystals by deformation along [001] in uniaxial compression at 1000°C. The {103} slip plane has been determined by slip trace analysis. Observations by transmission electron microscopy reveal the presence of screw dislocations with a Burgers vector of ½⟨331⟩. However, ½⟨331⟩ dislocations of mixed character lying along the ⟨ 010 ⟩ direction are observed to have decomposed into two other perfect dislocations by the energetically favourable reaction ½⟨ 331 ⟩→½⟨111⟩ + ⟨110⟩. The occurrence of such dislocation decomposition explains the difficulty of observing ½⟨331⟩ dislocations in previous work. The ½⟨331⟩ screw dislocations have been examined by weak-beam dark-field microscopy but dissociation into three ½⟨331⟩ collinear partials could not be detected.

Growth of DyBa2Cu3O7−x studied by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to investigate the microstructure of DyBa2Cu3O7−x films grown on (100) SrTiO3 by ozone-assisted molecular beam epitaxy. The surface roughness is found to be higher than that reported for sputtered films of YBa2Cu3O7−x. Intensity oscillations were observed in the specular spot of in situ reflection high energy electron diffraction patterns, a result which suggests that surface smoothness is not a requisite for the observation of such intensity oscillations. The observation of spiral morphology in STM pictures indicates the presence of screw dislocations, in excess of 109 cm−2. A zeroth order approximation has been used to evaluate the shapes of the spirals and reasonable agreement is obtained with experiment.

STUDY OF DISLOCATION ETCH PITS ON PbFe<sub>12</sub>O<sub>19</sub> SINGLE CRYSTALS

Using 50% HCI as etchant, we studied the morphology of dislocation etch pits on cleavaged (0001) basal faces of PbFe12O19 single crystals grown by the fluxed method. Three kinds of etch pits are found and two of them correspond to {1010} and {1011} types of dislocations, respectively. The third kind of etch pits is related to the presence of screw dislocations which are responsible for growth mechanism. Dislocation etch pits arrays observed on (0001) faces are interpreted by applying the mechanism proposed by Matthews et al.

Roughening transitions of finite order

It is shown that an asymmetric body-centered solid-on-solid model undergoes two roughening transitions, one of the Ising type ($\ensuremath{\alpha}=0$) the other with $\ensuremath{\alpha}=\ensuremath{-}2$. The model is further studied in the presence of screw dislocations and new evidence is provided for Kadanoff's conjecture that the Baxter line should appear in such a roughening model.

The dissolution of single crystals of hydroxyapatite in citric and lactic acids.

The dissolution of hydroxyapatite single crystals is highly anisotropic. Parallel to the c-axis the dissolution is fast, while perpendicular to the c-axis dissolution is negligible. In 5 M citric acid solutions at 37°, the acid penetrates into the single crystals parallel to the c-axis with a velocity of approximately 60 nM/sec. Scanning electron microscopy shows that the single crystals are twisted along the c-axis. Both the details of the dissolution behaviour as well as the surface morphology observed indicate the presence of screw dislocations in the single crystals with the dislocation line parallel to the c-axis.

Growth of Sucrose Crystals

LAYER growth on crystals was observed by Marcelin1 and other workers. Steps on crystals could be produced by surface nucleation or by the presence of screw dislocations which terminate at the surface2. The layers on cadmium iodide growing in aqueous solution3 were shown by Forty4 to be steps originating from dislocations. While Bunn3 was unable to find any layers on growing sucrose crystals, Powers5 observed them but without seeing any definite signs that they were due to screw dislocations. We have made an extensive examination of the surface features of sucrose crystals grown in the temperature-range 15–30° C. and conclude that the normal growth process is that described by Frank2. Step systems in the form of growth hills have been shown to be present on all faces of sucrose crystals when grown in pure sucrose solution and in sucrose solution with added dextrose or laevulose as impurity. Crystals grown in sucrose solution with added dextrose (0.2 per cent by weight of the sucrose) have steps which generally are much larger (about 40 times) than those grown in pure solutions of sucrose or in solutions with added laevulose.