X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism investigation of size effects on field-induced N\'eel-cap reversal

X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is used to investigate the influence of an applied magnetic field on N\'eel caps (i.e., surface terminations of asymmetric Bloch walls). Self-assembled micron-sized Fe(110) dots displaying a moderate distribution of size and aspect ratios serve as model objects. Investigations of remanent states after application of an applied field along the direction of N\'eel-cap magnetization give clear evidence for the magnetization reversal of the N\'eel caps around 120 mT, with a $\pm$20 mT dispersion. No clear correlation could be found between the value of the reversal field and geometrical features of the dots

Chiral nature of magnetic monopoles in artificial spin ice

Micromagnetic properties of monopoles in artificial kagome spin ice systems are investigated using numerical simulations. We show that micromagnetics brings additional complexity into the physics of these monopoles that is, by essence, absent in spin models: besides a fractionalized classical magnetic charge, monopoles in the artificial kagome ice are chiral at remanence. Our simulations predict that the chirality of these monopoles can be controlled without altering their charge state. This chirality breaks the vertex symmetry and triggers a directional motion of the monopole under an applied magnetic field. Our results also show that the choice of the geometrical features of the lattice can be used to turn on and off this chirality, thus allowing the investigation of chiral and achiral monopoles.Comment: 10 pages, 4 figure

High density QCD with static quarks

We study lattice QCD in the limit that the quark mass and chemical potential are simultaneously made large, resulting in a controllable density of quarks which do not move. This is similar in spirit to the quenched approximation for zero density QCD. In this approximation we find that the deconfinement transition seen at zero density becomes a smooth crossover at any nonzero density, and that at low enough temperature chiral symmetry remains broken at all densities.Comment: LaTeX, 18 pages, uses epsf.sty, postscript figures include

The roughness of stylolites: Implications of 3D high resolution topography measurements

Stylolites are natural pressure-dissolution surfaces in sedimentary rocks. We present 3D high resolution measurements at laboratory scales of their complex roughness. The topography is shown to be described by a self-affine scaling invariance. At large scales, the Hurst exponent is $\zeta_1 \approx 0.5$ and very different from that at small scales where $\zeta_2 \approx 1.2$. A cross-over length scale at around \L_c =1~mm is well characterized. Measurements are consistent with a Langevin equation that describes the growth of a stylolitic interface as a competition between stabilizing long range elastic interactions at large scales or local surface tension effects at small scales and a destabilizing quenched material disorder.Comment: 4 pages, 4 figure

The diet of harbour porpoises bycaught or washed ashore in Belgium, and relationship with relevant data from the strandings database

The harbour porpoise Phocoena phocoena is currently the most abundant marine mammal in Belgian waters. Large-scale changes in the distribution of this top predator in the North Sea have occurred during the last decades, possibly caused by changes in food availability. An analysis of the strandings database 1970-2011, containing data on 737 harbour porpoises, revealed that throughout the year two peaks in strandings occurred: one in spring, for an important part caused by the strandings of animals incidentally bycaught in fishing gear, and a second one during summer. Most of the stranded animals were juveniles, with a higher percentage of males than females. As could be expected, bycaught animals were on average heavier than equally sized naturally died animals, and they had a thicker blubber layer. In naturally died animals the blubber layer was thicker in winter than in summer, illustrating the insulating function of the blubber layer besides its function as an energy storage.We investigated the stomach content of 64 harbour porpoises washed ashore or bycaught in Belgium between 1997 and 2011. Ten of the stomachs were empty. Fish contributed to most of the prey remains. In total we found the remains of 19 fish species belonging to 10 families. The numerically most important prey items in juveniles were gobies (Gobiidae). Reconstructing the original weight of the prey items revealed that gobies constituted the most important prey by weight, but that larger sandeels (Ammodytidae) and to a lesser extent gadoids (Gadidae) were also important. In adults the majority of prey items were gobies and sandeels, but the reconstructed weight of the stomach content revealed that sandeels and gadoids constituted by far the most important prey. Surprisingly, clupeids (Clupeidae) did not contribute much to the diet, although the return of the harbour porpoise is often linked to an increase in herring Clupea harengu stocks in the southern North Sea. Also no twaite shad Alosa fallax were found, although this diadromic fish is common again in the area. A small number of smelt Osmerus eperlanus was found in some of the recently stranded harbour porpoises. Smelt is a diadromic fish of which densities are increasing from very low levels. The fact that juvenile harbour porpoises had apparently fed on large quantities of small bottom fish may help to explain why they were more prone to bycatch than adults. The analysis revealed that a gradual shift occurs in the feeding habits of harbour porpoises while becoming adults: from small benthic fish towards larger fish taken from the water column

Fracture Roughness Scaling: a case study on planar cracks

Using a multi-resolution technique, we analyze large in-plane fracture fronts moving slowly between two sintered Plexiglas plates. We find that the roughness of the front exhibits two distinct regimes separated by a crossover length scale $\delta^*$. Below $\delta^*$, we observe a multi-affine regime and the measured roughness exponent $\zeta_{\parallel}^{-} = 0.60\pm 0.05$ is in agreement with the coalescence model. Above $\delta^*$, the fronts are mono-affine, characterized by a roughness exponent $\zeta_{\parallel}^{+} = 0.35\pm0.05$, consistent with the fluctuating line model. We relate the crossover length scale to fluctuations in fracture toughness and the stress intensity factor

A Novel Approach for Ellipsoidal Outer-Approximation of the Intersection Region of Ellipses in the Plane

In this paper, a novel technique for tight outer-approximation of the intersection region of a finite number of ellipses in 2-dimensional (2D) space is proposed. First, the vertices of a tight polygon that contains the convex intersection of the ellipses are found in an efficient manner. To do so, the intersection points of the ellipses that fall on the boundary of the intersection region are determined, and a set of points is generated on the elliptic arcs connecting every two neighbouring intersection points. By finding the tangent lines to the ellipses at the extended set of points, a set of half-planes is obtained, whose intersection forms a polygon. To find the polygon more efficiently, the points are given an order and the intersection of the half-planes corresponding to every two neighbouring points is calculated. If the polygon is convex and bounded, these calculated points together with the initially obtained intersection points will form its vertices. If the polygon is non-convex or unbounded, we can detect this situation and then generate additional discrete points only on the elliptical arc segment causing the issue, and restart the algorithm to obtain a bounded and convex polygon. Finally, the smallest area ellipse that contains the vertices of the polygon is obtained by solving a convex optimization problem. Through numerical experiments, it is illustrated that the proposed technique returns a tighter outer-approximation of the intersection of multiple ellipses, compared to conventional techniques, with only slightly higher computational cost

The locality of the fourth root of staggered fermion determinant in the interacting case

The fourth root approximation in LQCD simulations with dynamical staggered fermions requires justification. We test its validity numerically in the interacting theory in a renormalization group framework.Comment: 6 pages, Talk presented at Lattice 2005 (Machines and Algorithms