Spontaneous nucleation of structural defects in inhomogeneous ion chains

Structural defects in ion crystals can be formed during a linear quench of the transverse trapping frequency across the mechanical instability from a linear chain to a zigzag structure. The density of defects after the sweep can be conveniently described by the Kibble-Zurek mechanism (KZM). In particular, the number of kinks in the zigzag ordering can be derived from a time-dependent Ginzburg-Landau equation for the order parameter, here the zigzag transverse size, under the assumption that the ions are continuously laser cooled. In a linear Paul trap, the transition becomes inhomogeneous, since the charge density is larger in the center and more rarefied at the edges. During the linear quench, the mechanical instability is first crossed in the center of the chain, and a front, at which the mechanical instability is crossed during the quench, is identified that propagates along the chain from the center to the edges. If the velocity of this front is smaller than the sound velocity, the dynamics become adiabatic even in the thermodynamic limit and no defect is produced. Otherwise, the nucleation of kinks is reduced with respect to the case in which the charges are homogeneously distributed, leading to a new scaling of the density of kinks with the quenching rate. The analytical predictions are verified numerically by integrating the Langevin equations of motion of the ions, in the presence of a time-dependent transverse confinement. We argue that the non-equilibrium dynamics of an ion chain in a Paul trap constitutes an ideal scenario to test the inhomogeneous extension of the KZM, which lacks experimental evidence to date.

Combined antenna and localized plasmon resonance in Raman scattering from Random arrays of silver-coated, vertically aligned multiwalled carbon nanotubes

The electric field enhancement associated with detailed structure within novel optical antenna nanostructures is modeled using the surface integral equation technique in the context of surface-enhanced Raman scattering (SERS). The antennae comprise random arrays of vertically aligned, multi-walled carbon nanotubes dressed with highly granular Ag. Different types of "hot-spot" underpinning the SERS are identified, but contrasting characteristics are revealed. Those at the outer edges of the Ag grains are antenna driven with field enhancement amplified in antenna antinodes while intergrain hotspots are largely independent of antenna activity. Hot-spots between the tops of antennae leaning towards each other also appear to benefit from antenna amplification.

Geophysical and geochemical survey of a large marine pockmark on the Malin Shelf, Ireland

Marine pockmarks are a specific type of seabed geological setting resembling craters or pits and are considered seabed surface expressions of fluid flow in the subsurface. A large composite pockmark on the Malin Shelf, off the northern coast of Ireland was surveyed and ground truthed to assess its activity and investigate fluid related processes in the subsurface. Geophysical (including acoustic and electromagnetic) data confirmed the subsurface presence of signatures typical of fluids within the sediment. Shallow seismic profiling revealed a large shallow gas pocket and typical gas related indicators such as acoustic blanking and enhanced reflectors present underneath and around the large pockmark. Sulphate profiles indicate that gas from the shallow reservoir has been migrating upwards, at least recently. However, there are no chimney structures observed in the sub-bottom data and the migration pathways are not apparent. Electromagnetic data show slightly elevated electrical conductivity on the edges of the pockmarks and a drop below regional levels within the confines of the pockmark, suggesting changes in physical properties of the sediment. Nuclear Magnetic Resonance (NMR) experiments were employed to characterize the organic component of sediments from selected depths. Very strong microbial signatures were evident in all NMR spectra but microbes outside the pockmark appear to be much more active than inside. These observations coincide with spikes in conductivity and the lateral gas bearing body suggesting that there is an increase in microbial activity and biomass when gas is present.

Observations of enhanced extreme ultraviolet continua during an X-class solar flare using SDO/EVE.

Observations of extreme ultraviolet (EUV) emission from an X-class solar flare that occurred on 2011 February 15 at 01: 44 UT are presented, obtained using the EUV Variability Experiment (EVE) on board the Solar Dynamics Observatory. The complete EVE spectral range covers the free-bound continua of H I (Lyman continuum), He I, and He II, with recombination edges at 91.2, 50.4, and 22.8 nm, respectively. By fitting the wavelength ranges blueward of each recombination edge with an exponential function, light curves of each of the integrated continua were generated over the course of the flare, as was emission from the free-free continuum (6.5-37 nm). The He II 30.4 nm and Ly alpha 121.6 nm lines, and soft X-ray (SXR; 0.1-0.8 nm) emission from GOES are also included for comparison. Each free-bound continuum was found to have a rapid rise phase at the flare onset similar to that seen in the 25-50 keV light curves from RHESSI, suggesting that they were formed by recombination with free electrons in the chromosphere. However, the free-free emission exhibited a slower rise phase seen also in the SXR emission from GOES, implying a predominantly coronal origin. By integrating over the entire flare the total energy emitted via each process was determined. We find that the flare energy in the EVE spectral range amounts to at most a few percent of the total flare energy, but EVE gives us a first comprehensive look at these diagnostically important continuum components.

Fast Retinal Vessel Detection and Measurement Using Wavelets and Edge Location Refinement

The relationship between changes in retinal vessel morphology and the onset and progression of diseases such as diabetes, hypertension and retinopathy of prematurity (ROP) has been the subject of several large scale clinical studies. However, the difficulty of quantifying changes in retinal vessels in a sufficiently fast, accurate and repeatable manner has restricted the application of the insights gleaned from these studies to clinical practice. This paper presents a novel algorithm for the efficient detection and measurement of retinal vessels, which is general enough that it can be applied to both low and high resolution fundus photographs and fluorescein angiograms upon the adjustment of only a few intuitive parameters. Firstly, we describe the simple vessel segmentation strategy, formulated in the language of wavelets, that is used for fast vessel detection. When validated using a publicly available database of retinal images, this segmentation achieves a true positive rate of 70.27%, false positive rate of 2.83%, and accuracy score of 0.9371. Vessel edges are then more precisely localised using image profiles computed perpendicularly across a spline fit of each detected vessel centreline, so that both local and global changes in vessel diameter can be readily quantified. Using a second image database, we show that the diameters output by our algorithm display good agreement with the manual measurements made by three independent observers. We conclude that the improved speed and generality offered by our algorithm are achieved without sacrificing accuracy. The algorithm is implemented in MATLAB along with a graphical user interface, and we have made the source code freely available. 

Lateral electron transport in high-intensity laser-irradiated foils diagnosed by ion emission

An experimental investigation of lateral electron transport in thin metallic foil targets irradiated by ultraintense (>= 10(19) W/cm(2)) laser pulses is reported. Two-dimensional spatially resolved ion emission measurements are used to quantify electric-field generation resulting from electron transport. The measurement of large electric fields (similar to 0.1 TV/m) millimeters from the laser focus reveals that lateral energy transport continues long after the laser pulse has decayed. Numerical simulations confirm a very strong enhancement of electron density and electric field at the edges of the target.

Measuring icon complexity: An automated analysis

Measures of icon designs rely heavily on surveys of the perceptions of population samples. Thus, measuring the extent to which changes in the structure of an icon will alter its perceived complexity can be costly and slow. An automated system capable of producing reliable estimates of perceived complexity could reduce development costs and time. Measures of icon complexity developed by Garcia, Badre, and Stasko (1994) and McDougall, Curry, and de Bruijn (1999) were correlated with six icon properties measured using Matlab (MathWorks, 2001) software, which uses image-processing techniques to measure icon properties. The six icon properties measured were icon foreground, the number of objects in an icon, the number of holes in those objects, and two calculations of icon edges and homogeneity in icon structure. The strongest correlates with human judgments of perceived icon complexity (McDougall et al., 1999) were structural variability (r(s) = .65) and edge information (r(s) =.64).

Electronic properties of interfaces and defects from many-body perturbation theory: Recent developments and applications

We review some recent developments in many body perturbation theory (MBPT) calculations that have enabled the study of interfaces and defects. Starting from the theoretical basis of MBPT, Hedin's equations are presented, leading to the CW and CWI' approximations. We introduce the perturbative approach, that is the one most commonly used for obtaining quasiparticle (QP) energies. The practical strategy presented for dealing with the frequency dependence of the self energy operator is based on either plasmon-pole models (PPM) or the contour deformation technique, with the latter being more accurate. We also discuss the extrapolar method for reducing the number of unoccupied states which need to be included explicity in the calculations. The use of the PAW method in the framework of MBPT is also described. Finally, results which have been obtained using, MBPT for band offsets a interfaces and for defects presented, with companies on the main difficulties and cancels.

Schematic representation of the QP corrections (marked with ) to the band edges (E and E-v) and a defect level (F) for a Si/SiO2 interface (Si and O atoms are represented in blue and red, respectively, in the ball and stick model) with an oxygen vacancy leading to a Si-Si bond (the Si atoms involved in this bond are colored light blue).

Single crystal CVD diamond growth strategy by the use of a 3D geometrical model: Growth on (113) oriented substrates

The quality of single crystal diamond obtained by microwave CVD processes has been drastically improved in the last 5 years thanks to surface pretreatment of the substrates [A. Tallaire, J. Achard, F. Silva, R.S. Sussmann, A. Gicquel, E. Rzepka, Physica Status Solidi (A) 201, 2419-2424 (2004); G. Bogdan, M. Nesladek, J. D'Haen, J. Maes, V.V. Moshchalkov, K. Haenen, M. D'Olieslaeger, Physica Status Solidi (A) 202, 2066-2072 (2005); M. Yamamoto, T. Teraji, T. Ito, Journal of Crystal Growth 285, 130-136 (2005)]. Additionally, recent results have unambiguously shown the occurrence of (110) faces on crystal edges and (113) faces on crystal corners [F. Silva, J. Achard, X. Bonnin, A. Michau, A. Tallaire, O. Brinza, A. Gicquel, Physica Status Solidi (A) 203, 3049-3055 (2006)]. We have developed a 3D geometrical growth model to account for the final crystal morphology. The basic parameters of this growth model are the relative displacement speeds of (111), (110) and (113) faces normalized to that of the (100) faces, respectively alpha, beta, and gamma. This model predicts both the final equilibrium shape of the crystal (i.e. after infinite growth time) and the crystal morphology as a function of alpha, beta, gamma, and deposition time.

An optimized operating point, deduced from the model, has been validated experimentally by measuring the growth rate in (100), (111), (110), and (113) orientations. Furthermore, the evolution of alpha, beta, gamma as a function of methane concentration in the gas discharge has been established. From these results, crystal growth strategies can be proposed in order, for example, to enlarge the deposition area. In particular, we will show, using the growth model, that the only possibility to significantly increase the deposition area is, for our growth conditions, to use a (113) oriented substrate. A comparison between the grown crystal and the model results will be discussed and characterizations of the grown film (Photoluminescence spectroscopy, EPR, SEM) will be presented. (C) 2008 Elsevier B.V. All rights reserved.

Arching or Compressive Membrane Action in Reinforced Concrete Slabs

Arching or compressive membrane action (CMA) in reinforced concrete slabs occurs as a result of the great difference between the tensile and compressive strength of concrete. Cracking of the concrete causes a migration of the neutral axis which is accompanied by in-plane expansion of the slab at its boundaries. If this natural tendency to expand is restrained, the development of arching action enhances the strength of the slab. The term arching action is normally used to describe the arching phenomenon in one-way spanning slabs and compressive membrane action is normally used to describe the arching phenomenon in two-
way spanning slabs. This encyclopedic article presents the background to the discovery of the phenomenon of arching action and presents a factual history of the approaches to the treatment of arching action in the United Kingdom and North American bridge deck design codes. The article summarises the theoretical methodology used in the United Kingdom Design Manual for Roads and Bridges, BD81/02, which was based on the work by Kirkpatrick, Rankin & Long at Queen's University Belfast.

Diabetic-like loss of corneal sensitivity in the 50% galactose fed rat is ameliorated with topical administration of an aldose reductase inhibitor

Purpose: To investigate the temporal course of corneal sensitivity loss & the role of aldose reductase inhibitors (ARI) in an animal model of diabetic ocular complications. Methods: Weanling male S-D rats were randomly grouped to received ad libitum water & diet consisting of Purina (#5001) w/ either: 50% starch (CON,n=15) or 50% D-galactose (GAL,n=30). Half the galactosemic rats (ARI,n=15) received topical 0.25% CT-112 (3x daily, 20µl, Senju Pharmaceutical Co., Japan). Control & remaining half of the galactosemic animals received equivalent doses of saline eyedrops. Rats were restrained w/o medication during sensitivity measurements conducted w/ a Cochet-Bonnet Aesthesiometer mounted on a micromanipulator. The end of the filament (0.012mm dia.), which applied a mean pressure of 0.96 g/mm perpendicular to the corneal surface at center, was in the plane of focus of a slit-lamp biomicroscope. Measurements were conducted by two investigators which were masked to the treatment group. The average blink-responses from 10 consecutive stimuli to each cornea were expressed as a percent. Results: Mean (±SD) baseline corneal sensitivity in all groups were similar (CON 73%±11, GAL 71%±15, ARI 74%±16). Corneal sensitivity in the galactosemic rat was decreased (p

Self-Similar Nested Flux Closure Structures in a Tetragonal Ferroelectric

In specific solid-state materials, under the right conditions, collections of magnetic dipoles are known to spontaneously form into a variety of rather complex geometrical patterns, exemplified by vortex and skyrmion structures. While theoretically, similar patterns should be expected to form from electrical dipoles, they have not been clearly observed to date: the need for continued experimental exploration is therefore clear. In this Letter we report the discovery of a rather complex domain arrangement that has spontaneously formed along the edges of a thin single crystal ferroelectric sheet, due to surface-related depolarizing fields. Polarization patterns are such that nanoscale “flux-closure” loops are nested within a larger mesoscale flux closure object. Despite the orders of magnitude differences in size, the geometric forms of the dual-scale flux closure entities are rather similar.

Gaussian pulse mixing and scattering by finite nonlinear layered structures

Multiple Gaussian pulse interactions and scattering in the nonlinear layered dielectric structures have been examined. The Gaussian pulses with different centre frequencies and lengths are incident at oblique angles on the finite stack of nonlinear dielectric layers. The properties of the reflected and refracted waveforms and the effects of the structure and the incident pulses' parameters on the mixing process are discussed. It is shown that the efficiency of forward emission at the combinatorial frequency can be considerably increased when the wavelengths of interacting pulses are close to the edges of electromagnetic bandgap. © 2012 IEEE.

GIANT LOBES OF CENTAURUS A RADIO GALAXY OBSERVED WITH THE SUZAKU X-RAY SATELLITE

We report on Suzaku observations of selected regions within the southern giant lobe of the radio galaxy Centaurus A. In our analysis we focus on distinct X-ray features detected with the X-ray Imaging Spectrometer within the range 0.5-10 keV, some of which are likely associated with fine structure of the lobe revealed by recent high-quality radio intensity and polarization maps. With the available photon statistics, we find that the spectral properties of the detected X-ray features are equally consistent with thermal emission from hot gas with temperatures kT > 1 keV, or with a power-law radiation continuum characterized by photon indices Gamma similar to 2.0 +/- 0.5. However, the plasma parameters implied by these different models favor a synchrotron origin for the analyzed X-ray spots, indicating that a very efficient acceleration of electrons up to greater than or similar to 10 TeV energies is taking place within the giant structure of Centaurus A, albeit only in isolated and compact regions associated with extended and highly polarized radio filaments. We also present a detailed analysis of the diffuse X-ray emission filling the whole field of view of the instrument, resulting in a tentative detection of a soft excess component best fitted by a thermal model with a temperature of kT similar to 0.5 keV. The exact origin of the observed excess remains uncertain, although energetic considerations point to thermal gas filling the bulk of the volume of the lobe and mixed with the non-thermal plasma, rather than to the alternative scenario involving a condensation of the hot intergalactic medium around the edges of the expanding radio structure. If correct, this would be the first detection of the thermal content of the extended lobes of a radio galaxy in X-rays. The corresponding number density of the thermal gas in such a case is n(g) similar to 10(-4) cm(-3), while its pressure appears to be in almost exact equipartition with the volume-averaged non-thermal pressure provided by the radio-emitting electrons and the lobes' magnetic field. A prominent large-scale fluctuation of the Galactic foreground emission, resulting in excess foreground X-ray emission aligned with the lobe, cannot be ruled out. Although tentative, our findings potentially imply that the structure of the extended lobes in active galaxies is likely to be highly inhomogeneous and non-uniform, with magnetic reconnection and turbulent acceleration processes continuously converting magnetic energy to internal energy of the plasma particles, leading to possibly significant spatial and temporal variations in the plasma beta parameter around the volume-averaged equilibrium condition beta similar to 1.