939 research outputs found
Influence of ion implantation on the magnetic and transport properties of manganite films
We have used oxygen ions irradiation to generate controlled structural
disorder in thin manganite films. Conductive atomic force microscopy CAFM),
transport and magnetic measurements were performed to analyze the influence of
the implantation process in the physical properties of the films. CAFM images
show regions with different conductivity values, probably due to the random
distribution of point defect or inhomogeneous changes of the local Mn3+/4+
ratio to reduce lattice strains of the irradiated areas. The transport and
magnetic properties of these systems are interpreted in this context.
Metal-insulator transition can be described in the frame of a percolative
model. Disorder increases the distance between conducting regions, lowering the
observed TMI. Point defect disorder increases localization of the carriers due
to increased disorder and locally enhanced strain field. Remarkably, even with
the inhomogeneous nature of the samples, no sign of low field magnetoresistance
was found. Point defect disorder decreases the system magnetization but doesn t
seem to change the magnetic transition temperature. As a consequence, an
important decoupling between the magnetic and the metal-insulator transition is
found for ion irradiated films as opposed to the classical double exchange
model scenario.Comment: 27 pages, 11 Figure
Skew-Unfolding the Skorokhod Reflection of a Continuous Semimartingale
The Skorokhod reflection of a continuous semimartingale is unfolded, in a
possibly skewed manner, into another continuous semimartingale on an enlarged
probability space according to the excursion-theoretic methodology of Prokaj
(2009). This is done in terms of a skew version of the Tanaka equation, whose
properties are studied in some detail. The result is used to construct a system
of two diffusive particles with rank-based characteristics and skew-elastic
collisions. Unfoldings of conventional reflections are also discussed, as are
examples involving skew Brownian Motions and skew Bessel processes.Comment: 20 pages. typos corrected, added a remark after Proposition 2.3,
simplified the last part of Example 2.
Information Loss in Coarse Graining of Polymer Configurations via Contact Matrices
Contact matrices provide a coarse grained description of the configuration
omega of a linear chain (polymer or random walk) on Z^n: C_{ij}(omega)=1 when
the distance between the position of the i-th and j-th step are less than or
equal to some distance "a" and C_{ij}(omega)=0 otherwise. We consider models in
which polymers of length N have weights corresponding to simple and
self-avoiding random walks, SRW and SAW, with "a" the minimal permissible
distance. We prove that to leading order in N, the number of matrices equals
the number of walks for SRW, but not for SAW. The coarse grained Shannon
entropies for SRW agree with the fine grained ones for n <= 2, but differs for
n >= 3.Comment: 18 pages, 2 figures, latex2e Main change: the introduction is
rewritten in a less formal way with the main results explained in simple
term
Random Planar Lattices and Integrated SuperBrownian Excursion
In this paper, a surprising connection is described between a specific brand
of random lattices, namely planar quadrangulations, and Aldous' Integrated
SuperBrownian Excursion (ISE). As a consequence, the radius r_n of a random
quadrangulation with n faces is shown to converge, up to scaling, to the width
r=R-L of the support of the one-dimensional ISE. More generally the
distribution of distances to a random vertex in a random quadrangulation is
described in its scaled limit by the random measure ISE shifted to set the
minimum of its support in zero.
The first combinatorial ingredient is an encoding of quadrangulations by
trees embedded in the positive half-line, reminiscent of Cori and Vauquelin's
well labelled trees. The second step relates these trees to embedded (discrete)
trees in the sense of Aldous, via the conjugation of tree principle, an
analogue for trees of Vervaat's construction of the Brownian excursion from the
bridge.
From probability theory, we need a new result of independent interest: the
weak convergence of the encoding of a random embedded plane tree by two contour
walks to the Brownian snake description of ISE.
Our results suggest the existence of a Continuum Random Map describing in
term of ISE the scaled limit of the dynamical triangulations considered in
two-dimensional pure quantum gravity.Comment: 44 pages, 22 figures. Slides and extended abstract version are
available at http://www.loria.fr/~schaeffe/Pub/Diameter/ and
http://www.iecn.u-nancy.fr/~chassain
Direct observation of electronic inhomogeneities induced by point defect disorder in manganite films
We have investigated the influence of point defect disorder in the electronic
properties of manganite films. Real-time mapping of ion irradiated samples
conductivity was performed though conductive atomic force microscopy (CAFM).
CAFM images show electronic inhomogeneities in the samples with different
physical properties due to spatial fluctuations in the point defect
distribution. As disorder increases, the distance between conducting regions
increases and the metal-insulator transition shifts to lower temperatures.
Transport properties in these systems can be interpreted in terms of a
percolative model. The samples saturation magnetization decreases as the
irradiation dose increases whereas the Curie temperature remains unchanged
The Baum-Connes Conjecture via Localisation of Categories
We redefine the Baum-Connes assembly map using simplicial approximation in
the equivariant Kasparov category. This new interpretation is ideal for
studying functorial properties and gives analogues of the assembly maps for all
equivariant homology theories, not just for the K-theory of the crossed
product. We extend many of the known techniques for proving the Baum-Connes
conjecture to this more general setting
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