207 research outputs found
Yukawa potentials in systems with partial periodic boundary conditions II : Lekner sums for quasi-two dimensional systems
Yukawa potentials may be long ranged when the Debye screening length is
large. In computer simulations, such long ranged potentials have to be taken
into account with convenient algorithms to avoid systematic bias in the
sampling of the phase space. Recently, we have provided Ewald sums for
quasi-two dimensional systems with Yukawa interaction potentials [M. Mazars,
{\it J. Chem. Phys.}, {\bf 126}, 056101 (2007) and M. Mazars, {\it Mol. Phys.},
Paper I]. Sometimes, Lekner sums are used as an alternative to Ewald sums for
Coulomb systems. In the present work, we derive the Lekner sums for quasi-two
dimensional systems with Yukawa interaction potentials and we give some
numerical tests for pratical implementations. The main result of this paper is
to outline that Lekner sums cannot be considered as an alternative to Ewald
sums for Yukawa potentials. As a conclusion to this work : Lekner sums should
not be used for quasi-two dimensional systems with Yukawa interaction
potentials.Comment: 25 pages, 5 figures and 1 tabl
Polarization of tightly focused laser beams
The polarization properties of monochromatic light beams are studied. In
contrast to the idealization of an electromagnetic plane wave, finite beams
which are everywhere linearly polarized in the same direction do not exist.
Neither do beams which are everywhere circularly polarized in a fixed plane. It
is also shown that transversely finite beams cannot be purely transverse in
both their electric and magnetic vectors, and that their electromagnetic energy
travels at less than c. The electric and magnetic fields in an electromagnetic
beam have different polarization properties in general, but there exists a
class of steady beams in which the electric and magnetic polarizations are the
same (and in which energy density and energy flux are independent of time).
Examples are given of exactly and approximately linearly polarized beams, and
of approximately circularly polarized beams.Comment: 9 pages, 6 figure
Evaluation of Coulomb potential in a triclinic cell with periodic boundary conditions
Lekner and Sperb's work on the evaluation of Coulomb energy and forces under
periodic boundary conditions is generalized that makes it possible to use a
triclinic unit cell in simulations in 3D rather than just an orthorhombic cell.
The expressions obtained are in a similar form as previously obtained by Lekner
and Sperb for the especial case of orthorhombic cell
A geometrical setting for the classification of multilayers
We elaborate on the consequences of the factorization of the transfer matrix
of any lossless multilayer in terms of three basic matrices of simple
interpretation. By considering the bilinear transformation that this transfer
matrix induces in the complex plane, we introduce the concept of multilayer
transfer function and study its properties in the unit disk. In this
geometrical setting, our factorization translates into three actions that can
be viewed as the basic pieces for understanding the multilayer behavior.
Additionally, we introduce a simple trace criterion that allows us to classify
multilayers in three types with properties closely related to one (and only
one) of these three basic matrices. We apply this approach to analyze some
practical examples that are representative of these types of matrices.Comment: 8 pages, 5 figures. To be published in J. Opt. Soc. Am.
Effective way to sum over long range Coulomb potentials in two and three dimensions
I propose a method to calculate logarithmic interaction in two dimensions and
coulomb interaction in three dimensions under periodic boundary conditions.
This paper considers the case of a rectangular cell in two dimensions and an
orthorhombic cell in three dimensions. Unlike the Ewald method, there is no
parameter to be optimized, nor does it involve error functions, thus leading to
the accuracy obtained. This method is similar in approach to that of Sperb [R.
Sperb, Mol. Simulation, 22, 199 (1999).], but the derivation is considerably
simpler and physically appealing. An important aspect of the proposed method is
the faster convergence of the Green function for a particular case as compared
to Sperb's work. The convergence of the sums for the most part of unit cell is
exponential, and hence requires the calculation of only a few dozen terms. In a
very simple way, we also obtain expressions for interaction for systems with
slab geometries. Expressions for the Madelung constant of CsCl and NaCl are
also obtained.Comment: To appear in Phy. Rev.
Polarization conversion spectroscopy of hybrid modes
Enhanced polarization conversion in reflection for the Otto and Kretschmann
configurations is introduced as a new method for hybrid-mode spectroscopy.
Polarization conversion in reflection appears when hybrid-modes are excited in
a guiding structure composed of at least one anisotropic media. In contrast to
a dark dip, in this case modes are associated to a peak in the converted
reflectance spectrum, increasing the detection sensitivity and avoiding
confusion with reflection dips associated with other processes as can be
transmission.Comment: 4 pages, 4 figure
Composition dependent magnetic properties of iron oxide - polyaniline nanoclusters
Gamma - Iron Oxide prepared by sol -gel process was used to produce
nanocomposites with polyaniline of varying aniline concentrations. TEM shows
the presence of chain like structure for lower polyaniline concentration. The
room temperature hysteresis curves show finite coercivity of 160 Oe for all the
composites while the saturation magnetization was found to decrease with
increasing polymer content. ZFC - FC magnetisation measurements indicate high
blocking temperatures. It is believed that this indicates a strongly
interacting system, which is also shown by our TEM results. Monte Carlo
simulations performed on a random anisotropy model with dipolar and exchange
inteactions match well with experimental results.Comment: 9 (nine) pages, 6 figures (jpeg and eps
Phase and transport velocities in particle and electromagnetic beams
In a coherent monoenergetic beam of non-interacting particles, the phase
velocity and the particle transport velocity are functions of position, with
the strongest variation being in the focal region. These velocities are
everywhere parallel to each other, and their product is constant in space. For
a coherent monochromatic electromagnetic beam, the energy transport velocity is
never greater than the speed of light, and can even be zero. The phase
velocities (one each for the non-zero components of the electric and magnetic
fields, in general) can be different from each other and from the energy
transport velocity, both in direction and in magnitude. The phase velocities at
a given point are independent of time, for both particle and electromagnetic
beams. The energy velocity is independent of time for the particle beam, but in
general oscillates (with angular frequency 2w) in magnitude and direction about
its mean value at a given point in the electromagnetic beam. However, there
exist electromagnetic steady beams, within which the energy flux, energy
density and energy velocity are all independent of time.Comment: 9 pages, 12 figure
Strong-Coupling Theory for Counter-Ion Distributions
The Poisson-Boltzmann approach gives asymptotically exact counter-ion density
profiles around charged objects in the weak-coupling limit of low valency and
high temperature. In this paper we derive, using field-theoretic methods, a
theory which becomes exact in the opposite limit of strong coupling. Formally,
it corresponds to a standard virial expansion. Long-range divergences, which
render the virial expansion intractable for homogeneous bulk systems, are shown
to be renormalizable for the case of inhomogeneous distribution functions by a
systematic expansion in inverse powers of the coupling parameter. For a planar
charged wall, our analytical results compare quantitatively with extensive
Monte-Carlo simulations.Comment: 7 pages, 3 figures; to appear in Europhys. Let
Optimizing omnidirectional reflection by multilayer mirrors
Periodic layered media can reflect strongly for all incident angles and
polarizations in a given frequency range. Quarter-wave stacks at normal
incidence are commonplace in the design of such omnidirectional reflectors. We
discuss alternative design criteria to optimize these systems.Comment: 9 pages, 6 figures. To be published in J. Opt. A: Pure and Applied
Optic
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