47,727 research outputs found
W-graph ideals
We introduce a concept of a W-graph ideal in a Coxeter group. The main goal
of this paper is to describe how to construct a W-graph from a given W-graph
ideal. The principal application of this idea is in type A, where it provides
an algorithm for the construction of W-graphs for Specht modules.Comment: 25 page
Strong disorder renormalization group on fractal lattices: Heisenberg models and magnetoresistive effects in tight binding models
We use a numerical implementation of the strong disorder renormalization
group (RG) method to study the low-energy fixed points of random Heisenberg and
tight-binding models on different types of fractal lattices. For the Heisenberg
model new types of infinite disorder and strong disorder fixed points are
found. For the tight-binding model we add an orbital magnetic field and use
both diagonal and off-diagonal disorder. For this model besides the gap spectra
we study also the fraction of frozen sites, the correlation function, the
persistent current and the two-terminal current. The lattices with an even
number of sites around each elementary plaquette show a dominant
periodicity. The lattices with an odd number of sites around each elementary
plaquette show a dominant periodicity at vanishing diagonal
disorder, with a positive weak localization-like magnetoconductance at infinite
disorder fixed points. The magnetoconductance with both diagonal and
off-diagonal disorder depends on the symmetry of the distribution of on-site
energies.Comment: 19 pages, 20 figure
Complexation of DNA with positive spheres: phase diagram of charge inversion and reentrant condensation
The phase diagram of a water solution of DNA and oppositely charged spherical
macroions is studied. DNA winds around spheres to form beads-on-a-string
complexes resembling the chromatin 10 nm fiber. At small enough concentration
of spheres these "artificial chromatin" complexes are negative, while at large
enough concentrations of spheres the charge of DNA is inverted by the adsorbed
spheres. Charges of complexes stabilize their solutions. In the plane of
concentrations of DNA and spheres the phases with positive and negative
complexes are separated by another phase, which contains the condensate of
neutral DNA-spheres complexes. Thus when the concentration of spheres grows,
DNA-spheres complexes experience condensation and resolubilization (or
reentrant condensation). Phenomenological theory of the phase diagram of
reentrant condensation and charge inversion is suggested. Parameters of this
theory are calculated by microscopic theory. It is shown that an important part
of the effect of a monovalent salt on the phase diagram can be described by the
nontrivial renormalization of the effective linear charge density of DNA wound
around a sphere, due to the Onsager-Manning condensation. We argue that our
phenomenological phase diagram or reentrant condensation is generic to a large
class of strongly asymmetric electrolytes. Possible implication of these
results for the natural chromatin are discussed.Comment: Many corrections to text. SUbmitted to J. Chem. Phy
Anomalous Hall Effect of Calcium-doped Lanthanum Cobaltite Films
The Hall resistivity, magnetoresistance, and magnetization of
La_{1-x}Ca_{x}CoO_{3} epitaxial films with x between 0.25 and 0.4 grown on
lanthanum aluminate were measured in fields up to 7 T. The x=1/3 film, shows a
reentrant metal insulator transition. Below 100 K, the x=1/3 and 0.4 films have
significant coercivity which increases with decreasing temperature. At low
temperature the Hall resistivity remains large and essentially field
independent in these films, except for a sign change at the coercive field that
is more abrupt than the switching of the magnetization. A unique
magnetoresistance behavior accompanies this effect. These results are discussed
in terms of a percolation picture and the mixed spin state model for this
system. We propose that the low-temperature Hall effect is caused by
spin-polarized carriers scattering off of orbital disorder in the spin-ordered
clusters.Comment: REVTeX 4, 3 pages with 4 encapsulated postscript graphics. Submitted
to MMM 2002 conference proceedings (J. Appl. Phys.
Variety and the evolution of refinery processing
Evolutionary theories of economic development stress the role of variety as both a determinant and a result of growth. In this paper we develop a measure of variety, based on Weitzman's maximum likelihood procedure. This measure is based on the distance between products, and indicates the degree of differentiation of a product population. We propose a generic method, which permits to regroup the products with very similar characteristics values before choosing randomly the product models to be used to calculate Weitzman's measure. We apply the variety measure to process characteristics of oil refining. The results obtained for this technology show classic evolutionary specialization patterns that can be understood on the basis of niche theory. Here the changes in variety are related to changes in the range of the services the technology considered can deliver, range which plays a role similar to that of the size of the habitat of a biological species.TECHNOLOGICAL EVOLUTION; REFINERY PROCESSES; NICHE THEORY; WEITZMAN MEASURE
Model for Anisotropic Directed Percolation
We propose a simulation model to study the properties of directed percolation
in two-dimensional (2D) anisotropic random media. The degree of anisotropy in
the model is given by the ratio between the axes of a semi-ellipse
enclosing the bonds that promote percolation in one direction. At percolation,
this simple model shows that the average number of bonds per site in 2D is an
invariant equal to 2.8 independently of . This result suggests that
Sinai's theorem proposed originally for isotropic percolation is also valid for
anisotropic directed percolation problems. The new invariant also yields a
constant fractal dimension for all , which is the same
value found in isotropic directed percolation (i.e., ).Comment: RevTeX, 9 pages, 3 figures. To appear in Phys.Rev.
Photo-assisted Andreev reflection as a probe of quantum noise
Andreev reflection, which corresponds to the tunneling of two electrons from
a metallic lead to a superconductor lead as a Cooper pair (or vice versa), can
be exploited to measure high frequency noise. A detector is proposed, which
consists of a normal lead--superconductor circuit, which is capacitively
coupled to a mesoscopic circuit where noise is to be measured. We discuss two
detector circuits: a single normal metal -- superconductor tunnel junction and
a normal metal separated from a superconductor by a quantum dot operating in
the Coulomb blockade regime. A substantial DC current flows in the detector
circuit when an appropriate photon is provided or absorbed by the mesoscopic
circuit, which plays the role of an environment for the junction to which it
couples. Results for the current can be cast in all cases in the form of a
frequency integral of the excess noise of the environment weighted by a kernel
which is specific to the transport process (quasiparticle tunneling, Andreev
reflection,...) which is considered. We apply these ideas to the measurement of
the excess noise of a quantum point contact and we provide numerical estimates
of the detector current.Comment: 19 pages, 11 figure
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