65,581 research outputs found
Angle-dependence of quantum oscillations in YBa2Cu3O6.59 shows free spin behaviour of quasiparticles
Measurements of quantum oscillations in the cuprate superconductors afford a
new opportunity to assess the extent to which the electronic properties of
these materials yield to a description rooted in Fermi liquid theory. However,
such an analysis is hampered by the small number of oscillatory periods
observed. Here we employ a genetic algorithm to globally model the field,
angular, and temperature dependence of the quantum oscillations observed in the
resistivity of YBa2Cu3O6.59. This approach successfully fits an entire data set
to a Fermi surface comprised of two small, quasi-2-dimensional cylinders. A key
feature of the data is the first identification of the effect of Zeeman
splitting, which separates spin-up and spin-down contributions, indicating that
the quasiparticles in the cuprates behave as nearly free spins, constraining
the source of the Fermi surface reconstruction to something other than a
conventional spin density wave with moments parallel to the CuO2 planes.Comment: 8 pages, 4 figure
Improvements to the construction of binary black hole initial data
Construction of binary black hole initial data is a prerequisite for
numerical evolutions of binary black holes. This paper reports improvements to
the binary black hole initial data solver in the Spectral Einstein Code, to
allow robust construction of initial data for mass-ratio above 10:1, and for
dimensionless black hole spins above 0.9, while improving efficiency for lower
mass-ratios and spins. We implement a more flexible domain decomposition,
adaptive mesh refinement and an updated method for choosing free parameters. We
also introduce a new method to control and eliminate residual linear momentum
in initial data for precessing systems, and demonstrate that it eliminates
gravitational mode mixing during the evolution. Finally, the new code is
applied to construct initial data for hyperbolic scattering and for binaries
with very small separation.Comment: 28 pages, 13 figures, 1 tabl
Micro-Macro relations for flow through random arrays of cylinders
The transverse permeability for creeping flow through unidirectional random arrays of fibers with various structures is revisited theoretically and numerically using the finite element method (FEM). The microstructure at various porosities has a strong effect on the transport properties, like permeability, of fibrous materials. We compare different microstructures (due to four random generator algorithms) as well as the effect of boundary conditions, finite size, homogeneity and isotropy of the structure on the macroscopic permeability of the fibrous medium. Permeability data for different minimal distances collapse when their minimal value is subtracted, which yields an empirical macroscopic permeability master function of porosity. Furthermore, as main result, a microstructural model is developed based on the lubrication effect in the narrow channels between neighboring fibers. The numerical experiments suggest a unique, scaling power law relationship between the permeability obtained from fluid flow simulations and the mean value of the shortest Delaunay triangulation edges (constructed using the centers of the fibers), which is identical to the averaged second nearest neighbor fiber distances. This universal lubrication relation, as valid in a wide range of porosities, accounts for the microstructure, e.g. hexagonally ordered or disordered fibrous media. It is complemented by a closure relation that relates the effective microscopic length to the packing fraction
Occlusion-related lateral connections stabilize kinetic depth stimuli through perceptual coupling
Local sensory information is often ambiguous forcing the brain to integrate spatiotemporally separated information for stable conscious perception. Lateral connections between clusters of similarly tuned neurons in the visual cortex are a potential neural substrate for the coupling of spatially separated visual information. Ecological optics suggests that perceptual coupling of visual information is particularly beneficial in occlusion situations. Here we present a novel neural network model and a series of human psychophysical experiments that can together explain the perceptual coupling of kinetic depth stimuli with activity-driven lateral information sharing in the far depth plane. Our most striking finding is the perceptual coupling of an ambiguous kinetic depth cylinder with a coaxially presented and disparity defined cylinder backside, while a similar frontside fails to evoke coupling. Altogether, our findings are consistent with the idea that clusters of similarly tuned far depth neurons share spatially separated motion information in order to resolve local perceptual ambiguities. The classification of far depth in the facilitation mechanism results from a combination of absolute and relative depth that suggests a functional role of these lateral connections in the perception of partially occluded objects
Counts-in-Cylinders in the Sloan Digital Sky Survey with Comparisons to N-body Simulations
Environmental statistics provide a necessary means of comparing the
properties of galaxies in different environments and a vital test of models of
galaxy formation within the prevailing, hierarchical cosmological model. We
explore counts-in-cylinders, a common statistic defined as the number of
companions of a particular galaxy found within a given projected radius and
redshift interval. Galaxy distributions with the same two-point correlation
functions do not necessarily have the same companion count distributions. We
use this statistic to examine the environments of galaxies in the Sloan Digital
Sky Survey, Data Release 4. We also make preliminary comparisons to four models
for the spatial distributions of galaxies, based on N-body simulations, and
data from SDSS DR4 to study the utility of the counts-in-cylinders statistic.
There is a very large scatter between the number of companions a galaxy has and
the mass of its parent dark matter halo and the halo occupation, limiting the
utility of this statistic for certain kinds of environmental studies. We also
show that prevalent, empirical models of galaxy clustering that match observed
two- and three-point clustering statistics well fail to reproduce some aspects
of the observed distribution of counts-in-cylinders on 1, 3 and 6-Mpc/h scales.
All models that we explore underpredict the fraction of galaxies with few or no
companions in 3 and 6-Mpc/h cylinders. Roughly 7% of galaxies in the real
universe are significantly more isolated within a 6 Mpc/h cylinder than the
galaxies in any of the models we use. Simple, phenomenological models that map
galaxies to dark matter halos fail to reproduce high-order clustering
statistics in low-density environments.Comment: 17 pages, 10 figures. Accepted, Ap
Mobility of Power-law and Carreau Fluids through Fibrous Media
The flow of generalized Newtonian fluids with a rate-dependent viscosity
through fibrous media is studied with a focus on developing relationships for
evaluating the effective fluid mobility. Three different methods have been used
here: i) a numerical solution of the Cauchy momentum equation with the Carreau
or power-law constitutive equations for pressure-driven flow in a fiber bed
consisting of a periodic array of cylindrical fibers, ii) an analytical
solution for a unit cell model representing the flow characteristics of a
periodic fibrous medium, and iii) a scaling analysis of characteristic bulk
parameters such as the effective shear rate, the effective viscosity,
geometrical parameters of the system, and the fluid rheology. Our scaling
analysis yields simple expressions for evaluating the transverse mobility
functions for each model, which can be used for a wide range of medium porosity
and fluid rheological parameters. While the dimensionless mobility is, in
general, a function of the Carreau number and the medium porosity, our results
show that for porosities less than , the dimensionless
mobility becomes independent of the Carreau number and the mobility function
exhibits power-law characteristics as a result of high shear rates at the pore
scale. We derive a suitable criterion for determining the flow regime and the
transition from a constant viscosity Newtonian response to a power-law regime
in terms of a new Carreau number rescaled with a dimensionless function which
incorporates the medium porosity and the arrangement of fibers
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