194 research outputs found
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
Een bladvlekkenziekte bij narcissen (Stagonospora curtisii) en mogelijkheden deze te bestrijden
Een aantasting van de bladeren van narcissen door de schimmel Stagonospora curtisii is in Engeland reeds lang bekend. De ziekte treedt daar voornamelijk op in narcissen die enkele jaren vast blijven staan en is vooral van economisch belang doordat de schimmel ook de bloemen aantast en waardeloos maakt. In Nederland worden de symptomen van deze ziekte op de bladeren van narcissen de laatste jaren in toenemende mate gevonden, ook veelvuldig bij trompetnarcissen als 'Golden Harvest' en 'Rembrandt'
Collineation group as a subgroup of the symmetric group
Let be the projectivization (i.e., the set of one-dimensional vector
subspaces) of a vector space of dimension over a field. Let be a
closed (in the pointwise convergence topology) subgroup of the permutation
group of the set . Suppose that contains the
projective group and an arbitrary self-bijection of transforming a
triple of collinear points to a non-collinear triple. It is well-known from
\cite{KantorMcDonough} that if is finite then contains the
alternating subgroup of .
We show in Theorem \ref{density} below that , if
is infinite.Comment: 9 page
A World-Volume Perspective on the Recombination of Intersecting Branes
We study brane recombination for supersymmetric configurations of
intersecting branes in terms of the world-volume field theory. This field
theory contains an impurity, corresponding to the degrees of freedom localized
at the intersection. The Higgs branch, on which the impurity fields condense,
consists of vacua for which the intersection is deformed into a smooth
calibrated manifold. We show this explicitly using a superspace formalism for
which the calibration equations arise naturally from F- and D-flatness.Comment: References adde
More on the Tensorial Central Charges in N=1 Supersymmetric Gauge Theories (BPS Wall Junctions and Strings)
We study the central extensions of the N=1 superalgebras relevant to the
soliton solutions with the axial geometry - strings, wall junctions, etc. A
general expression valid in any four-dimensional gauge theory is obtained. We
prove that the only gauge theory admitting BPS strings at weak coupling is
supersymmetric electrodynamics with the Fayet-Iliopoulos term. The problem of
ambiguity of the (1/2,1/2) central charge in the generalized Wess-Zumino models
and gauge theories with matter is addressed and solved. A possibility of
existence of the BPS strings at strong coupling in N=2 theories is discussed. A
representation of different strings within the brane picture is presented.Comment: 26 pages, 2 figures, 1 reference added, typos corrected, Sec. 9.3
expanded. Final version accepted for publication in Phys.Rev.
Limitations of the isolated GP-STN network
An in vitro mouse slice preparation from control and MPTP-treated mice in which functional reciprocal GP-STN connectivity is maintained, does not produce oscillatory bursting or synchronous activity neuronal activity. Pharmacological interventions that produce bursting activity do so without concomitant neuronal synchrony, or a requirement for glutamate or GABA transmission. Pre-treatment with MPTP did not alter this behaviour. Thus, we have no evidence that the functionally connected, but isolated, GP — STN network can act as a pacemaker for synchronous correlated activity in the basal ganglia and must conclude that other inputs such as those from cortex and/or striatum are required
Current-induced cooling phenomenon in a two-dimensional electron gas under a magnetic field
We investigate the spatial distribution of temperature induced by a dc
current in a two-dimensional electron gas (2DEG) subjected to a perpendicular
magnetic field. We numerically calculate the distributions of the electrostatic
potential phi and the temperature T in a 2DEG enclosed in a square area
surrounded by insulated-adiabatic (top and bottom) and isopotential-isothermal
(left and right) boundaries (with phi_{left} < phi_{right} and T_{left}
=T_{right}), using a pair of nonlinear Poisson equations (for phi and T) that
fully take into account thermoelectric and thermomagnetic phenomena, including
the Hall, Nernst, Ettingshausen, and Righi-Leduc effects. We find that, in the
vicinity of the left-bottom corner, the temperature becomes lower than the
fixed boundary temperature, contrary to the naive expectation that the
temperature is raised by the prevalent Joule heating effect. The cooling is
attributed to the Ettingshausen effect at the bottom adiabatic boundary, which
pumps up the heat away from the bottom boundary. In order to keep the adiabatic
condition, downward temperature gradient, hence the cooled area, is developed
near the boundary, with the resulting thermal diffusion compensating the upward
heat current due to the Ettingshausen effect.Comment: 25 pages, 7 figure
Kinky D-branes and straight strings of open string tachyon effective theory
In this letter we construct the kink D1-brane super D-helix solution and its
T-dual the D2-brane supertube using the effective action of non-BPS tachyonic
D-branes . In the limit of zero angular momentum, both types of solutions
collapse to zero radius, giving rise respectively to a degenerate string
configuration corresponding to a particle travelling with the speed of light
and to a static straight string configuration. These solutions share all the
properties of fundamental strings and do not have the pathological behavior of
other solutions previously found in this context. A short discussion on the
``generalized gauge transformations'' suggested by Sen is used to justify the
validity of our approach.Comment: 10 pages, latex, typos corrected and references adde
Basic Module Theory over Non-Commutative Rings with Computational Aspects of Operator Algebras
The present text surveys some relevant situations and results where basic
Module Theory interacts with computational aspects of operator algebras. We
tried to keep a balance between constructive and algebraic aspects.Comment: To appear in the Proceedings of the AADIOS 2012 conference, to be
published in Lecture Notes in Computer Scienc
Cosmology from Rolling Massive Scalar Field on the anti-D3 Brane of de Sitter Vacua
We investigate a string-inspired scenario associated with a rolling massive
scalar field on D-branes and discuss its cosmological implications. In
particular, we discuss cosmological evolution of the massive scalar field on
the ant-D3 brane of KKLT vacua. Unlike the case of tachyon field, because of
the warp factor of the anti-D3 brane, it is possible to obtain the required
level of amplitude of density perturbations. We study the spectra of scalar and
tensor perturbations generated during the rolling scalar inflation and show
that our scenario satisfies the observational constraint coming from the Cosmic
Microwave Background anisotropies and other observational data. We also
implement the negative cosmological constant arising from the stabilization of
the modulus fields in the KKLT vacua and find that this leads to a successful
reheating in which the energy density of the scalar field effectively scales as
a pressureless dust. The present dark energy can be also explained in our
scenario provided that the potential energy of the massive rolling scalar does
not exactly cancel with the amplitude of the negative cosmological constant at
the potential minimum.Comment: RevTex4, 15 pages, 5 eps figures, minor clarifications and few
references added, final version to appear in PR
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