16,590 research outputs found
Homogenization of Maxwell's equations in periodic composites
We consider the problem of homogenizing the Maxwell equations for periodic
composites. The analysis is based on Bloch-Floquet theory. We calculate
explicitly the reflection coefficient for a half-space, and derive and
implement a computationally-efficient continued-fraction expansion for the
effective permittivity. Our results are illustrated by numerical computations
for the case of two-dimensional systems. The homogenization theory of this
paper is designed to predict various physically-measurable quantities rather
than to simply approximate certain coefficients in a PDE.Comment: Significantly expanded compared to v1. Accepted to Phys.Rev.E. Some
color figures in this preprint may be easier to read because here we utilize
solid color lines, which are indistinguishable in black-and-white printin
Local nonsmooth Lyapunov pairs for first-order evolution differential inclusions
The general theory of Lyapunov's stability of first-order differential
inclusions in Hilbert spaces has been studied by the authors in a previous
work. This new contribution focuses on the natural case when the maximally
monotone operator governing the given inclusion has a domain with nonempty
interior. This setting permits to have nonincreasing Lyapunov functions on the
whole trajectory of the solution to the given differential inclusion. It also
allows some more explicit criteria for Lyapunov's pairs. Some consequences to
the viability of closed sets are given, as well as some useful cases relying on
the continuity or/and convexity of the involved functions. Our analysis makes
use of standard tools from convex and variational analysis
Homological stability for spaces of commuting elements in Lie groups
In this paper we study homological stability for spaces of pairwise commuting -tuples in a Lie group . We
prove that for each , these spaces satisfy rational homological
stability as ranges through any of the classical sequences of compact,
connected Lie groups, or their complexifications. We prove similar results for
rational equivariant homology, for character varieties, and for the
infinite-dimensional analogues of these spaces, and , introduced by Cohen-Stafa and Adem-Cohen-Torres-Giese
respectively. In addition, we show that the rational homology of the space of
unordered commuting -tuples in a fixed group stabilizes as
increases. Our proofs use the theory of representation stability - in
particular, the theory of -modules developed by
Church-Ellenberg-Farb and Wilson. In all of the these results, we obtain
specific bounds on the stable range, and we show that the homology isomorphisms
are induced by maps of spaces.Comment: 56 pages, accepted versio
Finite-Difference Time-Domain Study of Guided Modes in Nano-plasmonic Waveguides
A conformal dispersive finite-difference time-domain (FDTD) method is
developed for the study of one-dimensional (1-D) plasmonic waveguides formed by
an array of periodic infinite-long silver cylinders at optical frequencies. The
curved surfaces of circular and elliptical inclusions are modelled in
orthogonal FDTD grid using effective permittivities (EPs) and the material
frequency dispersion is taken into account using an auxiliary differential
equation (ADE) method. The proposed FDTD method does not introduce numerical
instability but it requires a fourth-order discretisation procedure. To the
authors' knowledge, it is the first time that the modelling of curved
structures using a conformal scheme is combined with the dispersive FDTD
method. The dispersion diagrams obtained using EPs and staircase approximations
are compared with those from the frequency domain embedding method. It is shown
that the dispersion diagram can be modified by adding additional elements or
changing geometry of inclusions. Numerical simulations of plasmonic waveguides
formed by seven elements show that row(s) of silver nanoscale cylinders can
guide the propagation of light due to the coupling of surface plasmons.Comment: 6 pages, 10 figures, accepted for publication, IEEE Trans. Antennas
Propaga
Modelling of Phase Separation in Alloys with Coherent Elastic Misfit
Elastic interactions arising from a difference of lattice spacing between two
coherent phases can have a strong influence on the phase separation
(coarsening) of alloys. If the elastic moduli are different in the two phases,
the elastic interactions may accelerate, slow down or even stop the phase
separation process. If the material is elastically anisotropic, the
precipitates can be shaped like plates or needles instead of spheres and can
form regular precipitate superlattices. Tensions or compressions applied
externally to the specimen may have a strong effect on the shapes and
arrangement of the precipitates. In this paper, we review the main theoretical
approaches that have been used to model these effects and we relate them to
experimental observations. The theoretical approaches considered are (i)
`macroscopic' models treating the two phases as elastic media separated by a
sharp interface (ii) `mesoscopic' models in which the concentration varies
continuously across the interface (iii) `microscopic' models which use the
positions of individual atoms.Comment: 106 pages, in Latex, figures available upon request, e-mail
addresses: [email protected], [email protected],
[email protected], submitted to the Journal of Statistical Physic
Regimes of heating and dynamical response in driven many-body localized systems
We explore the response of many-body localized (MBL) systems to periodic
driving of arbitrary amplitude, focusing on the rate at which they exchange
energy with the drive. To this end, we introduce an infinite-temperature
generalization of the effective "heating rate" in terms of the spread of a
random walk in energy space. We compute this heating rate numerically and
estimate it analytically in various regimes. When the drive amplitude is much
smaller than the frequency, this effective heating rate is given by linear
response theory with a coefficient that is proportional to the optical
conductivity; in the opposite limit, the response is nonlinear and the heating
rate is a nontrivial power-law of time. We discuss the mechanisms underlying
this crossover in the MBL phase, and comment on its implications for the
subdiffusive thermal phase near the MBL transition.Comment: 17 pages, 9 figure
The structure and stability of persistence modules
We give a self-contained treatment of the theory of persistence modules
indexed over the real line. We give new proofs of the standard results.
Persistence diagrams are constructed using measure theory. Linear algebra
lemmas are simplified using a new notation for calculations on quiver
representations. We show that the stringent finiteness conditions required by
traditional methods are not necessary to prove the existence and stability of
the persistence diagram. We introduce weaker hypotheses for taming persistence
modules, which are met in practice and are strong enough for the theory still
to work. The constructions and proofs enabled by our framework are, we claim,
cleaner and simpler.Comment: New version. We discuss in greater depth the interpolation lemma for
persistence module
Commuting matrices and Atiyah's Real K-theory
We describe the -equivariant homotopy type of the space of commuting
n-tuples in the stable unitary group in terms of Real K-theory. The result is
used to give a complete calculation of the homotopy groups of the space of
commuting n-tuples in the stable orthogonal group, as well as of the
coefficient ring for commutative orthogonal K-theory.Comment: Minor changes. To appear in Journal of Topolog
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