1,464 research outputs found
On the existence of flat orthogonal matrices
In this note we investigate the existence of flat orthogonal matrices, i.e.
real orthogonal matrices with all entries having absolute value close to
. Entries of correspond to
Hadamard matrices, so the question of existence of flat orthogonal matrices can
be viewed as a relaxation of the Hadamard problem.Comment: 10 page
Intrinsic aging and effective viscosity in the slow dynamics of a soft glass with tunable elasticity
We investigate by rheology and light scattering the influence of the elastic
modulus, , on the slow dynamics and the aging of a soft glass. We show
that the slow dynamics and the aging can be entirely described by the evolution
of an effective viscosity, , defined as the characteristic time
measured in a stress relaxation experiment times . At all time,
is found to be independent of , of elastic perturbations, and
of the rate at which the sample is quenched in the glassy phase. We propose a
simple model that links to the internal stress built up at the
fluid-to-solid transition
Enumeration of three term arithmetic progressions in fixed density sets
Additive combinatorics is built around the famous theorem by Szemer\'edi
which asserts existence of arithmetic progressions of any length among the
integers. There exist several different proofs of the theorem based on very
different techniques. Szemer\'edi's theorem is an existence statement, whereas
the ultimate goal in combinatorics is always to make enumeration statements. In
this article we develop new methods based on real algebraic geometry to obtain
several quantitative statements on the number of arithmetic progressions in
fixed density sets. We further discuss the possibility of a generalization of
Szemer\'edi's theorem using methods from real algebraic geometry.Comment: 62 pages. Update v2: Corrected some references. Update v3:
Incorporated feedbac
Microscopic Model for High-spin vs. Low-spin ground state in () magnetic clusters
Conventional superexchange rules predict ferromagnetic exchange interaction
between Ni(II) and M (M=Mo(V), W(V), Nb(IV)). Recent experiments show that in
some systems this superexchange is antiferromagnetic. To understand this
feature, in this paper we develop a microscopic model for Ni(II)-M systems and
solve it exactly using a valence bond approach. We identify the direct exchange
coupling, the splitting of the magnetic orbitals and the inter-orbital electron
repulsions, on the M site as the parameters which control the ground state spin
of various clusters of the Ni(II)-M system. We present quantum phase diagrams
which delineate the high-spin and low-spin ground states in the parameter
space. We fit the spin gap to a spin Hamiltonian and extract the effective
exchange constant within the experimentally observed range, for reasonable
parameter values. We also find a region in the parameter space where an
intermediate spin state is the ground state. These results indicate that the
spin spectrum of the microscopic model cannot be reproduced by a simple
Heisenberg exchange Hamiltonian.Comment: 8 pages including 7 figure
Dispersions of ellipsoidal particles in a nematic liquid crystal
Colloidal particles dispersed in a partially ordered medium, such as a liquid
crystal (LC) phase, disturb its alignment and are subject to elastic forces.
These forces are long-ranged, anisotropic and tunable through temperature or
external fields, making them a valuable asset to control colloidal assembly.
The latter is very sensitive to the particle geometry since it alters the
interactions between the colloids. We here present a detailed numerical
analysis of the energetics of elongated objects, namely prolate ellipsoids,
immersed in a nematic host. The results, complemented with qualitative
experiments, reveal novel LC configurations with peculiar topological
properties around the ellipsoids, depending on their aspect ratio and the
boundary conditions imposed on the nematic order parameter. The latter also
determine the preferred orientation of ellipsoids in the nematic field, because
of elastic torques, as well as the morphology of particles aggregates.Comment: 31 pages, 11 figure
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