15,013 research outputs found
Develop and test fuel cell powered on site integrated total energy sysems: Phase 3: Full-scale power plant development
A 25-cell stack of the 13 inch x 23 inch cell size (about 4kW) remains on test after 8300 hours, using simulated reformate fuel. A similar stack was previously shut down after 7000 hours on load. These tests have been carried out for the purpose of assessing the durability of fuel cell stack components developed through the end of 1983. A 25kW stack containing 175 cells of the same size and utilizing a technology base representative of the 25-cell stacks has been constructed and is undergoing initial testing. A third 4kW stack is being prepared, and this stack will incorporate several new technology features
Driven vortices in 3D layered superconductors: Dynamical ordering along the c-axis
We study a 3D model of driven vortices in weakly coupled layered
superconductors with strong pinning. Above the critical force , we find a
plastic flow regime in which pancakes in different layers are uncoupled,
corresponding to a pancake gas. At a higher , there is an ``smectic flow''
regime with short-range interlayer order, corresponding to an entangled line
liquid. Later, the transverse displacements freeze and vortices become
correlated along the c-axis, resulting in a transverse solid. Finally, at a
force the longitudinal displacements freeze and we find a coherent solid
of rigid lines.Comment: 4 pages, 3 postscript figure
Moving Wigner Glasses and Smectics: Dynamics of Disordered Wigner Crystals
We examine the dynamics of driven classical Wigner solids interacting with
quenched disorder from charged impurities. For strong disorder, the initial
motion is plastic -- in the form of crossing winding channels. For increasing
drive, the disordered Wigner glass can reorder to a moving Wigner smectic --
with the electrons moving in non-crossing 1D channels. These different dynamic
phases can be related to the conduction noise and I(V) curves. For strong
disorder, we show criticality in the voltage onset just above depinning. We
also obtain the dynamic phase diagram for driven Wigner solids and prove that
there is a finite threshold for transverse sliding, recently found
experimentally.Comment: 4 pages, 4 postscript figure
Braided Rivers and Superconducting Vortex Avalanches
Magnetic vortices intermittently flow through preferred channels when they
are forced in or out of a superconductor. We study this behavior using a
cellular model, and find that the vortex flow can make braided rivers
strikingly similar to aerial photographs of braided fluvial rivers, such as the
Brahmaputra. By developing an analysis technique suitable for characterizing a
self-affine (multi)fractal, the scaling properties of the braided vortex rivers
in the model are compared with those of braided fluvial rivers. We suggest that
avalanche dynamics leads to braiding in both cases.Comment: 4 pages, 3 figures. To appear in PR
Changes in union membership over time : a panel analysis for West Germany
Despite the apparent stability of the wage bargaining institutions in West Germany, aggregate union membership has been declining dramatically since the early 90's. However, aggregate gross membership numbers do not distinguish by employment status and it is impossible to disaggregate these sufficiently. This paper uses four waves of the German Socioeconomic Panel in 1985, 1989, 1993, and 1998 to perform a panel analysis of net union membership among employees. We estimate a correlated random effects probit model suggested in Chamberlain (1984) to take proper account of individual specfic effects. Our results suggest that at the individual level the propensity to be a union member has not changed considerably over time. Thus, the aggregate decline in membership is due to composition effects. We also use the estimates to predict net union density at the industry level based on the IAB employment subsample for the time period 1985 to 1997. JEL - Klassifikation: J
Topos theory and `neo-realist' quantum theory
Topos theory, a branch of category theory, has been proposed as mathematical
basis for the formulation of physical theories. In this article, we give a
brief introduction to this approach, emphasising the logical aspects. Each
topos serves as a `mathematical universe' with an internal logic, which is used
to assign truth-values to all propositions about a physical system. We show in
detail how this works for (algebraic) quantum theory.Comment: 22 pages, no figures; contribution for Proceedings of workshop
"Recent Developments in Quantum Field Theory", MPI MIS Leipzig, July 200
Flux front penetration in disordered superconductors
We investigate flux front penetration in a disordered type II superconductor
by molecular dynamics (MD) simulations of interacting vortices and find scaling
laws for the front position and the density profile. The scaling can be
understood performing a coarse graining of the system and writing a disordered
non-linear diffusion equation. Integrating numerically the equation, we observe
a crossover from flat to fractal front penetration as the system parameters are
varied. The value of the fractal dimension indicates that the invasion process
is described by gradient percolation.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
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