Coherent and Incoherent Vortex Flow States in Crossed Channels

We examine vortex flow states in periodic square pinning arrays with one row and one column of pinning sites removed to create an easy flow crossed channel geometry. When a drive is simultaneously applied along both major symmetry axes of the pinning array such that vortices move in both channels, a series of coherent flow states develop in the channel intersection at rational ratios of the drive components in each symmetry direction when the vortices can cross the intersection without local collisions. The coherent flow states are correlated with a series of anomalies in the velocity force curves, and in some cases can produce negative differential conductivity. The same general behavior could also be realized in other systems including colloids, particle traffic in microfluidic devices, or Wigner crystals in crossed one-dimensional channels.Comment: 5 pages, 4 postscript figure

Evaluation of the effects of recent nature development measures in the Yser Estuary on ground beetle and spider assemblages

Since 1990, populations of ground beetles and spiders are continuously monitored in the coastal dune and saltmarsh habitats of the Yser Estuary (Nieuwpoort), within the context of a long-term study on invertebrate diversity, population dynamics and ecological and genetic effects of habitat fragmentation. By now, we know in detail the faunal composition of practically all available habitats in the study area. This is an ideal framework to monitor the effects of the nature development activities that started in 2001 in the area. Results of five large sampling campaigns in 2001-2003 (more than 25.000 carabid beetles and spiders, 218 species) show a number of new ground beetle and spider species and assemblages, but warrant that many of these could be rapidly lost again

Extractibilityof Residual Rock and Superphosphate on Limed and Unlimed Bungor Soil.

The possibility of using soil extractants to monitor the transformation of phosphorus (P) fertilizer on a Bungor series soil was investigated. Three soil extractants - Bray 2, Ammonium acetate lactate (AAL) and Olsen - were evaluated. The soil was treated with three levels of P each from rock phosphate and superphosphate at 0, 150 and 300kg Plha, no lime and limed at 4 tons/ha and a P fertilizer application interval of one year. Bray 2 and AAL were able to detect recent P from aged P applications with a higher amount of P being extracted from the recent. These extracts were able to differentiate between the 150 and 300kg Plha treatments but were not able to distinguish the forms of P. Olsen extractant on the other hand was able to detect the two P forms and showed P release from rock phosphate with time

Commensurate and Incommensurate Vortex States in Superconductors with Periodic Pinning Arrays

As a function of applied field, we find a rich variety of ordered and partially-ordered vortex lattice configurations in systems with square or triangular arrays of pinning sites. We present formulas that predict the matching fields at which commensurate vortex configurations occur and the vortex lattice orientation with respect to the pinning lattice. Our results are in excellent agreement with recent imaging experiments on square pinning arrays [K. Harada et al., Science 274, 1167 (1996)].Comment: 9 pages, 3 figures. Accepted to Physical Review

Confinement and Quantization Effects in Mesoscopic Superconducting Structures

We have studied quantization and confinement effects in nanostructured superconductors. Three different types of nanostructured samples were investigated: individual structures (line, loop, dot), 1-dimensional (1D) clusters of loops and 2D clusters of antidots, and finally large lattices of antidots. Hereby, a crossover from individual elementary "plaquettes", via clusters, to huge arrays of these elements, is realized. The main idea of our study was to vary the boundary conditions for confinement of the superconducting condensate by taking samples of different topology and, through that, modifying the lowest Landau level E_LLL(H). Since the critical temperature versus applied magnetic field T_c(H) is, in fact, E_LLL(H) measured in temperature units, it is varied as well when the sample topology is changed through nanostructuring. We demonstrate that in all studied nanostructured superconductors the shape of the T_c(H) phase boundary is determined by the confinement topology in a unique way.Comment: 28 pages, 19 EPS figures, uses LaTeX's aipproc.sty, contribution to Euroschool on "Superconductivity in Networks and Mesoscopic Systems", held in Siena, Italy (8-20 september 1997

Charge characteristics in relation to free iron and organic matter of soils from Bambouto Mountains, Western Cameroon

We have examined the charge characteristics, with special emphasis on the role of free Fe and organic matter, of humid tropical soils from Bambouto Mountains, Western Cameroon. The soils, which are formed from tuff, basalt and trachyte, are dominated by kaolinite and sesquioxides. The mounts of Fe oxides in them increase somewhat with depth. Open 2:1 phyllosilicates are present in trace amounts. The point of zero charge of the variable charge components, pH0, is around 4 in the topsoil (0-20 cm) and around 6 at 100-150 cm depth. In the subsoils, pH0 exceeds soil pH presumably because of large quantities of Fe oxides. Deferration increases both soil pH and pH0, but diminishes the anion exchange capacity. Oxides and oxyhydrates of Fe have positive surface charge, so their removal from the soils would result in overall loss of positive charge. Increases in soil pH would bring about an increase in the cation exchange capacity of the soils. Hence, management practices that reduce soil acidity should reduce loss of essential basic cations via leaching