209,264 research outputs found
Guided resonances in photonic crystals with point-defected aperiodically-ordered supercells
In this paper, we study the excitation of guided resonances (GRs) in
photonic-crystal slabs based on point-defected aperiodically-ordered
supercells. With specific reference to perforated-slab structures and the
Ammann-Beenker octagonal lattice geometry, we carry out full-wave numerical
studies of the plane-wave responses and of the underlying modal structures,
which illustrate the representative effects induced by the introduction of
symmetry-preserving and symmetry-breaking defects. Our results demonstrate that
breaking the supercell mirror symmetries via the judicious introduction of
point-defects enables for the excitation of otherwise uncoupled GRs, with
control on the symmetry properties of their field distributions, thereby
constituting an attractive alternative to those GR-engineering approaches based
on the asymmetrization of the hole shape. In this framework,
aperiodically-ordered supercells seem to be inherently suited, in view of the
variety of inequivalent defect sites that they can offer.Comment: 13 pages, 12 figures, 1 table. Slight change in the title; major
changes in the text and figure
Thermal stability of metastable magnetic skyrmions: Entropic narrowing and significance of internal eigenmodes
We compute annihilation rates of metastable magnetic skyrmions using a form
of Langer's theory in the intermediate-to-high damping (IHD) regime. For a
N\'eel skyrmion, a Bloch skyrmion, and an antiskyrmion, we look at two possible
paths to annihilation: collapse and escape through a boundary. We also study
the effects of a curved vs. a flat boundary, a second skyrmion and a
non-magnetic defect. We find that the skyrmion's internal modes play a dominant
role in the thermally activated transitions compared to the spin-wave
excitations and that the relative contribution of internal modes depends on the
nature of the transition process. Our calculations for a small skyrmion
stabilized at zero-field show that collapse on a defect is the most probable
path. In the absence of a defect, the annihilation is largely dominated by
escape mechanisms, even though in this case the activation energy is higher
than that of collapse processes. Escape through a flat boundary is found more
probable than through a curved boundary. The potential source of stability of
metastable skyrmions is therefore found not to lie in high activation energies,
nor in the dynamics at the transition state, but comes from entropic narrowing
in the saddle point region which leads to lowered attempt frequencies. This
narrowing effect is found to be primarily associated with the skyrmion's
internal modes.Comment: 14 pages, 9 figure
Dopant-Dopant Interactions in Beryllium doped Indium Gallium Arsenide: an Ab Initio Study
We present an ab initio study of dopant-dopant interactions in
beryllium-doped InGaAs. We consider defect formation energies of various
interstitial and substitutional defects and their combinations. We find that
all substitutional-substitutional interactions can be neglected. On the other
hand, interactions involving an interstitial defect are significant. Specially,
interstitial Be is stabilized by about 0.9/1.0 eV in the presence of one/two
BeGa substitutionals. Ga interstitial is also substantially stabilized by Be
interstitials. Two Be interstitials can form a metastable Be-Be-Ga complex with
a dissociation energy of 0.26 eV/Be. Therefore, interstitial defects and
defect-defect interactions should be considered in accurate models of Be doped
InGaAs. We suggest that In and Ga should be treated as separate atoms and not
lumped into a single effective group III element, as has been done before. We
identified dopant-centred states which indicate the presence of other charge
states at finite temperatures, specifically, the presence of Beint+1 (as
opposed to Beint+2 at 0K)
Hybridization of Bayesian networks and belief functions to assess risk. Application to aircraft deconstruction
This paper aims to present a study on knowledge management for the disassembly of end-of-life aircraft. We propose a model using Bayesian networks to assess risk and present three approaches to integrate the belief functions standing for the representation of fuzzy and uncertain knowledge
Computer simulations of the interactions of the (012) and (001) surfaces of jarosite with Al, Cd, Cu2+ and Zn
Jarosite is an important mineral on Earth, and possibly on Mars, where it controls the mobility of iron, sulfate and potentially toxic metals. Atomistic simulations have been used to study the incorporation of Al3+, and the M2+ impurities Cd, Cu and Zn, in the (0 1 2) and (0 0 1) surfaces of jarosite. The calculations show that the incorporation of Al on an Fe site is favorable on all surfaces in which terminal Fe ions are exposed, and especially on the (0 0 1) [Fe3(OH)3]6+ surface. Incorporation of Cd, Cu or Zn on a K site balanced by a K vacancy is predicted to stabilize the surfaces, but calculated endothermic solution energies and the high degree of distortion of the surfaces following incorporation suggest that these substitutions will be limited. The calculations also suggest that incorporation of Cd, Cu and Zn on an Fe site balanced by an OH vacancy, or by coupled substitution on both K and Fe sites, is unfavorable, although this might be compensated for by growth of a new layer of jarosite or goethite, as predicted for bulk jarosite. The results of the simulations show that surface structure will exert an influence on uptake of impurities in the order Cu > Cd > Zn, with the most favorable surfaces for incorporation being (0 1 2) [KFe(OH)4]0 and (0 0 1) [Fe3(OH)3]6+
Self healing of vacancy defects in single layer graphene and silicene
Self healing mechanisms of vacancy defects in graphene and silicene are
studied using first principles calculations. We investigated host adatom
adsorption, diffusion, vacancy formation and revealed atomistic mechanisms in
the healing of single, double and triple vacancies of single layer graphene and
silicene. Silicon adatom, which is adsorbed to silicene at the top site forms a
dumbbell like structure by pushing one Si atom underneath. The asymmetric
reconstruction of the single vacancy in graphene is induced by the
magnetization through the rebonding of two dangling bonds and acquiring a
significant magnetic moment through remaining unsaturated dangling bond. In
silicene, three two-fold coordinated atoms surrounding the single vacancy
become four-fold coordinated and nonmagnetic through rebonding. The energy
gained through new bond formation becomes the driving force for the
reconstruction. Under the external supply of host atoms, while the vacancy
defects of graphene heal perfectly, Stone-Wales defect can form in the course
of healing of silicene vacancy. The electronic and magnetic properties of
suspended, single layer graphene and silicene are modified by reconstructed
vacancy defects.Comment: Published in PRB: http://prb.aps.org/abstract/PRB/v88/i4/e04544
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The influence of heterogenous porosity on silicon nitride/steel wear in lubricated rolling contact
Heterogeneous porosity is detected on the surface and subsurface of hot isostatically pressed (HIPed) silicon nitride spherical rolling elements. The extent of the localised porosity accounts for an area of 6% of the rolling element surface and 4% of the material volume. An experimental investigation using a rotary tribometer is described to compare the lubricated rolling wear mechanisms and performance of HIPed silicon nitride with heterogeneous porosity defect in contact with steel. A brief review of previous investigations is presented. Localised porosity detection using white and violet light microscopy with post-test evaluation is described. Discussions, micro-hardness measurements and scanning electron microscopy illustrations are presented. Critical localised porosity size is evaluated from experimental results
Point defect dynamics in bcc metals
We present an analysis of the time evolution of self-interstitial atom and
vacancy (point defect) populations in pure bcc metals under constant
irradiation flux conditions. Mean-field rate equations are developed in
parallel to a kinetic Monte Carlo (kMC) model. When only considering the
elementary processes of defect production, defect migration, recombination and
absorption at sinks, the kMC model and rate equations are shown to be
equivalent and the time evolution of the point defect populations is analyzed
using simple scaling arguments. We show that the typically large mismatch of
the rates of interstitial and vacancy migration in bcc metals can lead to a
vacancy population that grows as the square root of time. The vacancy cluster
size distribution under both irreversible and reversible attachment can be
described by a simple exponential function. We also consider the effect of
highly mobile interstitial clusters and apply the model with parameters
appropriate for vanadium and iron.Comment: to appear in Phys. Rev.
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