1,110 research outputs found
Critical point for the strong field magnetoresistance of a normal conductor/perfect insulator/perfect conductor composite with a random columnar microstructure
A recently developed self-consistent effective medium approximation, for
composites with a columnar microstructure, is applied to such a
three-constituent mixture of isotropic normal conductor, perfect insulator, and
perfect conductor, where a strong magnetic field {\bf B} is present in the
plane perpendicular to the columnar axis. When the insulating and perfectly
conducting constituents do not percolate in that plane, the
microstructure-induced in-plane magnetoresistance is found to saturate for
large {\bf B}, if the volume fraction of the perfect conductor is greater
than that of the perfect insulator . By contrast, if , that
magnetoresistance keeps increasing as without ever saturating. This
abrupt change in the macroscopic response, which occurs when , is a
critical point, with the associated critical exponents and scaling behavior
that are characteristic of such points. The physical reasons for the singular
behavior of the macroscopic response are discussed. A new type of percolation
process is apparently involved in this phenomenon.Comment: 4 pages, 1 figur
High field magnetotransport in composite conductors: the effective medium approximation revisited
The self consistent effective medium approximation (SEMA) is used to study
three-dimensional random conducting composites under the influence of a strong
magnetic field {\bf B}, in the case where all constituents exhibit isotropic
response. Asymptotic analysis is used to obtain almost closed form results for
the strong field magnetoresistance and Hall resistance in various types of two-
and three-constituent isotropic mixtures for the entire range of compositions.
Numerical solutions of the SEMA equations are also obtained, in some cases, and
compared with those results. In two-constituent
free-electron-metal/perfect-insulator mixtures, the magnetoresistance is
asymptotically proportional to at {\em all concentrations above the
percolation threshold}. In three-constituent metal/insulator/superconductor
mixtures a line of critical points is found, where the strong field
magnetoresistance switches abruptly from saturating to non-saturating
dependence on , at a certain value of the
insulator-to-superconductor concentration ratio. This transition appears to be
related to the phenomenon of anisotropic percolation.Comment: 16 pages, 3 figure
Grackles
Numbering in the tens of millions of birds, grackle populations in North America can cause a variety of conflicts with people. Grackles eat agricultural crops and livestock feed, damage property, spread pathogens, and collide with aircraft. Their large roosts can be a nuisance in urban and suburban areas. A combination of dispersal techniques, exclusion, and lethal removal may help to reduce grackle damage.
Grackles adapt easily to human-dominated environments, and exploit human food and other features of human landscapes. Thus, an integrated damage management approach to grackle damage focuses on reducing and eliminating the damage, rather than simply controlling grackle populations.
Three species of grackles are present in North America: the common grackle, the boat-tailed grackle, and the great-tailed grackle. A fourth species, the greater Antillean grackle (Q. niger) is present in Puerto Rico. All are part of the Family Icteridae that includes blackbirds, orioles, cowbirds, meadowlarks, and bobolinks. The population status of all three grackle species is considered common to overabundant
New Method to Calculate Electrical Forces Acting on a Sphere in an Electrorheological Fluid
We describe a method to calculate the electrical force acting on a sphere in
a suspension of dielectric spheres in a host with a different dielectric
constant, under the assumption that a spatially uniform electric field is
applied. The method uses a spectral representation for the total electrostatic
energy of the composite. The force is expressed as a certain gradient of this
energy, which can be expressed in a closed analytic form rather than evaluated
as a numerical derivative. The method is applicable even when both the spheres
and the host have frequency-dependent dielectric functions and nonzero
conductivities, provided the system is in the quasistatic regime. In principle,
it includes all multipolar contributions to the force, and it can be used to
calculate multi-body as well as pairwise forces. We also present several
numerical examples, including host fluids with finite conductivities. The force
between spheres approaches the dipole-dipole limit, as expected, at large
separations, but departs drastically from that limit when the spheres are
nearly in contact. The force may also change sign as a function of frequency
when the host is a slightly conducting fluid.Comment: 29 pages, 8 figures, Accepted for Publication in Physical Review
Magnetoresistance of Three-Constituent Composites: Percolation Near a Critical Line
Scaling theory, duality symmetry, and numerical simulations of a random
network model are used to study the magnetoresistance of a
metal/insulator/perfect conductor composite with a disordered columnar
microstructure. The phase diagram is found to have a critical line which
separates regions of saturating and non-saturating magnetoresistance. The
percolation problem which describes this line is a generalization of
anisotropic percolation. We locate the percolation threshold and determine the
t = s = 1.30 +- 0.02, nu = 4/3 +- 0.02, which are the same as in
two-constituent 2D isotropic percolation. We also determine the exponents which
characterize the critical dependence on magnetic field, and confirm numerically
that nu is independent of anisotropy. We propose and test a complete scaling
description of the magnetoresistance in the vicinity of the critical line.Comment: Substantially revised version; description of behavior in finite
magnetic fields added. 7 pages, 7 figures, submitted to PR
Grackles
Numbering in the tens of millions of birds, grackle populations in North America can cause a variety of conflicts with people. Grackles eat agricultural crops and livestock feed, damage property, spread pathogens, and collide with aircraft. Their large roosts can be a nuisance in urban and suburban areas. A combination of dispersal techniques, exclusion, and lethal removal may help to reduce grackle damage.
Grackles adapt easily to human-dominated environments, and exploit human food and other features of human landscapes. Thus, an integrated damage management approach to grackle damage focuses on reducing and eliminating the damage, rather than simply controlling grackle populations.
Three species of grackles are present in North America: the common grackle, the boat-tailed grackle, and the great-tailed grackle. A fourth species, the greater Antillean grackle (Q. niger) is present in Puerto Rico. All are part of the Family Icteridae that includes blackbirds, orioles, cowbirds, meadowlarks, and bobolinks. The population status of all three grackle species is considered common to overabundant
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