Effective Core Radius Research Articles

Multiscale modeling of dislocations: combining peridynamics with gradient elasticity

Modeling dislocations is an inherently multiscale problem as one needs to simultaneously describe the high stress fields near the dislocation cores, which depend on atomistic length scales, and a surface boundary value problem which depends on boundary conditions on the sample scale. We present a novel approach which is based on a peridynamic dislocation model to deal with the surface boundary value problem. In this model, the singularity of the stress field at the dislocation core is regularized owing to the non-local nature of peridynamics. The effective core radius is defined by the peridynamic horizon which, for reasons of computational cost, must be chosen much larger than the lattice constant. This implies that dislocation stresses in the near-core region are seriously underestimated. By exploiting relationships between peridynamics and Mindlin-type gradient elasticity, we then show that gradient elasticity can be used to construct short-range corrections to the peridynamic stress field that yield a correct description of dislocation stresses from the atomic to the sample scale.

Soft core fluid with competing interactions at a hard wall

A model of competing pair interaction in the form of a three-Yukawa (3Y) potential is considered. The results of computer simulations show that the model proposed can describe spontaneous appearance in a homogeneous fluid of a variety of mesostructured phases. Furthermore, these self-assembly effects appear to be favored by the presence of confining walls. A field theory approach is subsequently applied to a mesoscopically homogeneous 3Y fluid at a hard wall. Explicit analytical expressions for the pair correlation function and the density profile are derived. The structure factor is calculated by introducing an effective hard core radius characteristic of the system, based on which the λ-lines is constructed. It is shown that the theoretical results obtained for density profiles of a 3Y fluid near a hard wall are in a good agreement with computer simulations data.

Numerical Simulation of Electromagnetic Wave Interaction with Spheroidal Core-Shell Nanoparticle: Dependence of Surface Plasmon Resonance on Core-Shell Composition

In the present work, we report our observations drawn from the numerical simulation of absorption and scattering efficiencies of spheroid shape nanostructures using discrete dipole approximation technique. The absorption and scattering spectra are discussed in terms of shell thickness, composition of core and shell material, aspect ratio and size of the spheroidal nanoparticle (NP). The normalized electric field intensity around the sphere and spheroidal core@shell geometry has been studied in the present work. The characteristics of surface plasmon resonance (SPR) depend upon the symmetry of NP. For core@shell spherical nanogeometry, only one plasmon mode is found while for asymmetric nanogeometry (such as oblate and prolate), more distinct plasmonic modes are found. It was accounted that for oblate shape TiO2@Au spheroid of effective core radius 80 nm with 0.1 aspect ratio, two SPR peaks have been found, one is in visible range (725 nm) corresponding to transverse mode (TM) and the other is in an infrared region (1099 nm) corresponding to longitudinal mode (LM). In this paper, two cases of core-shell oblate shape nanogeometry are considered to study the optical properties; one is nanospheroid of metal (Au, Ag) shell with an oxide (TiO2) core, and the other is metal core with an oxide shell. Numerical results illustrate that the behaviour of SPR of these two core@shell structures are quite different from each other. On increasing the shell thickness of metal@dielectric nanogeometry, SPR peak is red shifted while blue shifted for dielectric@metal nanogeometry. The comparison has been done between analytical (electrostatic approach) and numerical (DDA) approaches for ellipsoidal nanogeometries. It was observed that numerical results are in good agreement with analytical predictions for very small sized NP (d < < λ).

Near wake development of a wing tip vortex under the effect of synthetic jet actuation

An experimental investigation on the effectiveness of synthetic jet actuation in controlling a wingtip vortex of a rectangular, square-tipped NACA 0012 wing at a chord Reynolds number Rec=8×104 is reported. Spanwise pressure measurements on the suction surface of the wing showed that forcing the wingtip vortex at a frequency in the range of frequencies of the long and short-wave instabilities resulted in decreased pressure coefficients as compared to natural vortex measurements. Two control configurations were then considered for hotwire wake measurements, namely, a “least effective” configuration with momentum coefficient Cμ=0.004 and actuation frequency F+=0.71, and a “most effective” configuration with Cμ=0.04 and F+=0.213. Measurements at x/c=1 showed that, under the latter case, the vortex was stretched into an ellipsoid shape with a 50% average decrease in the peak tangential velocity, and 30% broadening of the effective vortex core radius. Further downstream, the vortex seemed to regain its universal shape but with reduced strength. The results suggested that the lower frequency control configuration allowed the synthetic jet to travel larger distances into the vortex bringing turbulent structures within its core resulting in increased mixing and subsequently decreased vortex strength.

Radiation chemical yields for the losses of typical functional groups in PADC films for high energy protons registered as unetchable tracks

A series of FT-IR spectrometric studies has been performed to understand the latent track structure in poly(allyl diglycol carbonate), PADC, which were exposed to proton beams with energies of 20, 30 and 70 MeV. These energies are too high to register etchable tracks in PADC. Chemical damage parameters, such as damage density, effective track core radius and radiation chemical yields, for losses of ether bond, carbonate ester bond and CH groups in PADC are evaluated as a function of the stopping power, which were compared to the previous results for 5.7 MeV proton and heavy ions, between He and Xe. Graphs of the chemical damage parameters are given at the wide stopping powers ranging from 1 to 12,000 keV/μm. The decreasing behaviors of the ether and carbonate ester bonds are on the almost identical trends with those of the heavy ions. On the contrary to this, the reducing behavior of CH groups is similar to that of the gamma rays. Different dependence of the chemical damage parameters for the loss of CH groups is found on the stopping powers between the both sides of the detection threshold as an etched track detector.

Sizes and shapes of young star cluster light profiles in M83

We measure the radii and two-dimensional light profiles of a large sample of young, massive star clusters in M83 using archival HST/WFC3 imaging of seven adjacent fields. We use GALFIT to fit the two-dimensional light profiles of the clusters, from which we find effective (half-light) radii, core radii, and slopes of the power-law (EFF) profile ($\eta$). We find lognormal distributions of effective radius and core radius, with medians of $\approx$2.5 pc and $\approx$1.3 pc, respectively. Our results provide strong evidence for a characteristic size of young, massive clusters. The average effective radius and core radius increase somewhat with cluster age. Little to no change in effective radius is observed with increasing galactocentric distance, except perhaps for clusters younger than 100 Myr. We find a shallow correlation between effective radius and mass for the full cluster sample, but a stronger correlation is present for clusters 200-300 Myr in age. Finally, the majority of the clusters are best fit by an EFF model with index $\eta\leq3.0$. There is no strong evidence for change in $\eta$ with cluster age, mass, or galactocentric distance. Our results suggest that clusters emerge from early evolution with similar radii and are not strongly affected by the tidal field of M83. Mass loss due to stellar evolution and/or GMC interactions appear to dominate cluster expansion in the age range we study.

Atomistically enabled nonsingular anisotropic elastic representation of near-core dislocation stress fields inα-iron

The stress fields of dislocations predicted by classical elasticity are known to be unrealistically large approaching the dislocation core, due to the singular nature of the theory. While in many cases this is remedied with the approximation of an effective core radius, inside which ad hoc regularizations are implemented, such approximations lead to a compromise in the accuracy of the calculations. In this work, an anisotropic non-singular elastic representation of dislocation fields is developed to accurately represent the near-core stresses of dislocations in $\alpha$-iron. The regularized stress field is enabled through the use of a non-singular Green's tensor function of Helmholtz-type gradient anisotropic elasticity, which requires only a single characteristic length parameter in addition to the material's elastic constants. Using a novel magnetic bond-order potential to model atomic interactions in iron, molecular statics calculations are performed, and an optimization procedure is developed to extract the required length parameter. Results show the method can accurately replicate the magnitude and decay of the near-core dislocation stresses even for atoms belonging to the core itself. Comparisons with the singular isotropic and anisotropic theories show the non-singular anisotropic theory leads to a substantially more accurate representation of the stresses of both screw and edge dislocations near the core, in some cases showing improvements in accuracy of up to an order of magnitude. The spatial extent of the region in which the singular and non-singular stress differ substantially is also discussed. The general procedure we describe may in principle be applied to accurately model the near-core dislocation stresses of any arbitrarily shaped dislocation in anisotropic cubic media.

Geometric definition of the V-parameter in photonic crystal fibers

We propose a general geometric definition of the V-parameter in photonic crystal fibers (PCF). We argue that the fundamental modal field in both square-lattice and triangular-lattice PCFs can be approached by certain proper assemblies of the fundamental mode in a one-dimensional planar waveguide. This observation suggests a geometric definition of the effective core radius and the single-mode regime of V≤π/2 in various PCFs. The numerical results imply that the suggestion is valid for all of the one-rod, seven-rod, 19-rod, and 37-rod core triangular-lattice PCFs and the one-rod, nine-rod, and 25-rod core square-lattice PCFs.

Comparison of Certain Local Pseudopotentials and a New Proposal

A new proposal of a local pseudopotential is put forwarded here depending on the concept of extended core radius in which half of the nearest neighbour distance is treated as effective core radius. There is no input of any property for fitting this parameter in this formalism. This pseudopotential is found to satisfy all the necessary requirements for applications. With this model potential we have evaluated the form factors for several bcc, fcc and hcp metals and achieved excellent agreement with previous results. On the same footing, we have examined other 14 local pseudo potentials also and on the basis of the comparison, the presently proposed pseudopotential is found to be much better. As a test case study, we have evaluated phonon dispersion curves of some liquid metals, viz. Na (Z = 1), Mg (Z = 2), Al (Z= 3) and Pb (Z = 4) and obtained quite satisfactory results.

V-parameter Study of Silica-Air 1D Photonic Crystal Fiber by Modulating Geometrical Parameters at Different Optical Communication Ranges

-Electromagnetic wave propagation inside one-dimensional photonic crystal fiber is numerically characterized by computing V-parameter, cladding index and effective mode profiles. Dependence on these parameters on geometrical structure of the fiber reflects the design tolerance, and corresponding modal characteristics. Air-hole spacing and periodicity are varied to observe the effect on V-parameter; along with effective core radius, which is a function of periodicity, is modified to see the effect on cladding index and effective mode profiles. Results obtained from empirical relations method are compared with analytical formula of Nielsen and Mortensen. Periodicity of the structure is regarded as numerically equal to the central wavelength of the desired range throughout the computation. Refractive index of the core is taken as independent of operating wavelength. Simulated parameters have critical importance to estimate the characteristics of photonic crystal fiber which will help the design engineers to study the performance at 870 nm, 1330 nm and 1550 nm. Keywords--V-Parameter Cladding Index, Effective Core Radius, Single Mode Propagation, Empirical Relations Method, Optical Communication

A theoretical study of large solid-core square-lattice silica photonic crystal fibers with square air-holes

Large solid-core fiber designs constructed by omitting nine air-holes at the center of square-lattice silica photonic crystals with square shaped air-holes have been investigated theoretically and their characteristics have been compared with those of circular ones. The V-parameter was estimated by using the step-index fiber approximation and the effective mode area calculated for the structures considered. To estimate the effective core radius of the structure with square shaped air-holes an approximation based on the air-filling fraction was used. The dispersion and confinement losses were also obtained and compared for both of the structures.

Effect of an external magnetic field on a critical point for phase separation in a dusty plasma

The effect of an external magnetic field on a critical point for phase transitions in a dusty plasma is investigated. It is shown that the ambient magnetic field increases the effective hard core radius of dust particles, which, in turn, would affect a critical point in terms of the Coulomb coupling parameter and the ratio between the inter-dust grain spacing and the dusty plasma Debye radius. The present result may be useful in understanding the phenomenon of liquid–vapor phase transitions in laboratory dusty plasmas that are held in an external magnetic field.

Applicability of the polyimide films as an SSNTD material

Abstract Track registration properties in polyimide films, KAPTON, for heavy ions have been examined by means of FT-IR spectrometry and the chemical etching in sodium hypochlorite solution. The effective track core size for the loss of C O and C–N–C composing imide bonds, and diphenyl ethers of C–O–C have been evaluated under the irradiations by Ne, Fe and Xe ions at energies less than 6 MeV/n. On the other hand, the etching property of the polyimide films has been examined in the sodium hypochlorite solution at temperature of 55 °C. Before the etchings, the films were exposed to H, C, Ne, Fe and Xe ions, at incident energies below 6 MeV/n. The etch pits are found only on the films exposed to Fe and Xe ions, indicating significant difference on the etch pit size between them. This implies that the polyimide film has charge or energy resolution for these relatively heavy ions. The threshold level of the etchable track registration is inferred to be around 2500 keV/μm. The effective track core radius at this stopping power for the loss of diphenyl ether is 1.6 nm, which is equivalent to the length between the adjacent diphenyl ether bonds in the polyimide chains. Breakings at two adjacent diphenyl ethers in radial direction of latent tracks may produce etchable tracks in KAPTON.

Solid-core square-lattice photonic crystal fibers: comparative studies of the single-mode regime and numerical aperture for circular and square air-holes

In this paper we have studied solid-core square-lattice PCFs with circular and square shaped air-holes. We have estimated the V-parameter by using the step-index fiber approximation (SIFA) and calculated numerical apertures for the structures considered. In order to estimate the effective core radius of the structure with the square shaped air-holes we have used an approximation based on the air-filling fraction.

Radiation chemical yields for loss of ether and carbonate ester bonds in PADC films exposed to proton and heavy ion beams

Abstract Chemical modification along ion tracks in PADC films has been studied by means of FT-IR spectrometry, which was exposed to proton and heavy ions of He, C, Ne, Ar, Fe, Kr and Xe with energies around the Bragg peaks. This study covers a wide region of the stopping power ranging from 10 to 10,000 keV/μm. Removal cross sections for the loss of ether and carbonate ester bonds are assessed for each ion, as a function of the stopping power. Chemical damage parameters like the damage density, the effective track core radius and the radiation chemical yields, G values (scissions/100 eV), for each bond are also derived. We have found anomalous dependence of these parameters on the stopping power. The G value for the loss of carbonate ester bond decreased from 20 for proton down to 5 for C and Ne ions, and then increased with atomic number of heavy ions up to 8 for Xe ion. Radial dose distribution for each ion has been also calculated. Results are discussed from the viewpoint of polymeric structure of PADC that consists of two parts with different radio-sensitivities.

The effect of temperature on elastic constants and bulk modulus

A model pseudopotential depending on an effective core radius is proposed to study the elastic constants C11, C12, C44 and bulk modulus B of Cu, Ag, Au, Ni, Pt and Rh at various temperatures (300–1500 K). The contribution of d-like electron is taken into account by introducing repulsive short-range Born-Mayer term. An excellent agreement between theoretical investigation and experimental findings has confirmed our formalism.

Improved fully vectorial effective index method for photonic crystal fibers: evaluation and enhancement

In the effective index method, which models photonic crystal fibers by means of a step-index waveguide analogy, two critical parameters are the effective index of the cladding and the effective core radius. It has been shown that the use of an effective core radius as a function of the relative air hole diameter, or also of the relative wavelength, can improve the accuracy of this method. We show, by comparison with a rigorous finite-difference frequency-domain method, that the reported improved fully vectorial effective index methods have commonly adopted a radius of the equivalent circular unit cell, which does not give the best accurate effective cladding index as compared with the use of an equivalent circular unit cell having the same area as the hexagonal unit cell. Furthermore, by defining both the radius of the equivalent circular unit cell and the effective core radius as a function of the relative air hole diameter, and the relative wavelength, we believe that the fully vectorial effective index method can be further enhanced in terms of accuracy of both the effective cladding index and the modal index.

Analysis and Design of Photonic Crystal Fibers Based on an Improved Effective-Index Method

The modal characteristics of photonic crystal fibers (PCFs), with guiding cores consisting of one or seven missing airholes, are investigated with the finite element method and compared to those of step-index fibers (SIFs). To extend the applicability of the classical SIF theories to PCFs, the effective refractive index of photonic crystal cladding and the effective core radius of a PCF are studied systematically, based on simple physically consistent concepts. With the new effective cladding index and core radius of PCFs, the classical definition of the V parameter for SIFs is extended to PCFs, and a highly efficient approach based on the effective-index method is developed for the design of PCFs. The new design approach has been successfully employed to analyze the modal properties of PCF lasers with depressed-index cores and further tested by using it to effectively estimate the number of guided modes for PCFs with large cores

Core structure, dislocation energy and Peierls stress for [formula omitted] edge dislocations with (0 0 0 1) and [formula omitted] slip planes in α-Zr

Atomic-scale simulations of edge dislocations of the 1 / 3 1 1 2 ¯ 0 ( 0 0 0 1 ) and 1 / 3 1 1 2 ¯ 0 { 1 1 ¯ 0 0 } slip systems have been carried out using a Finnis-Sinclair-type interatomic potential for α-zirconium. The distribution of atomic displacements in the dislocation core shows that in this model the edge dislocation in the basal plane dissociates into two Shockley partials whereas the dislocation in the prism plane remains undissociated. The effective core radius and core energy are estimated, and dislocation response to increasing applied shear strain is investigated. The core properties and the critical stress for dislocation glide (Peierls stress) depend sensitively on whether the core extends or not.

Unified study of electrical resistivity of simple and non-simple liquid metals

A model potential depending on an effective core radius but otherwise parameter free is used for the comparative study of electrical resistivity of simple and non-simple liquid metals. In the present paper electrical resistivity of simple and non-simple liquid metals have been calculated using Ziman’s formula, Ziman’s formula modified and used by Khajil and Tomak (Self consistent approximation) and t-matrix formulation given by Evans and Evans et al. Previously no one have reported such comparative study using pseudopotentials. In the electrical resistivity calculations we have used structure factor derived through charge hard sphere approximation. The beauty of this approximation is that it needs pseudopotential form factor for the calculation of structure factor. So this gives the better explanation of structure factor than any other approximations. From present investigations it is found that self consistent formulation results are excellently agrees with the experimental findings. A successful application is an evidence that our potential can predict wide class of physical properties of d and f shell metals as well as simple and non-simple metals.