41,944 research outputs found
Evaluation and study of advanced optical contamination, deposition, measurement, and removal techniques
A program is described to design, fabricate and install an experimental work chamber assembly (WCA) to provide a wide range of experimental capability. The WCA incorporates several techniques for studying the kinetics of contaminant films and their effect on optical surfaces. It incorporates the capability for depositing both optical and contaminant films on temperature-controlled samples, and for in-situ measurements of the vacuum ultraviolet reflectance. Ellipsometer optics are mounted on the chamber for film thickness determinations, and other features include access ports for radiation sources and instrumentation. Several supporting studies were conducted to define specific chamber requirements, to determine the sensitivity of the measurement techniques to be incorporated in the chamber, and to establish procedures for handling samples prior to their installation in the chamber. A bibliography and literature survey of contamination-related articles is included
Non-equilibrium condensation and coarsening of field-driven dipolar colloids
In colloidal suspensions, self-organization processes can be easily fueled by
external fields. One particularly interesting class of phenomena occurs in
monolayers of dipolar particles that are driven by rotating external fields.
Here we report results from a computer simulation study of such systems
focusing on the clustering behavior also observed in recent experiments. The
key result of this paper is a novel interpretation of this pattern formation
phenomenon: We show the clustering to be a by-product of a vapor-liquid first
order phase transition. In fact, the observed dynamic coarsening process
corresponds to the spindodal demixing that occurs during such a transitionComment: 6 pages, 5 figure
Field-induced structure transformation in electrorheological solids
We have computed the local electric field in a body-centered tetragonal (BCT)
lattice of point dipoles via the Ewald-Kornfeld formulation, in an attempt to
examine the effects of a structure transformation on the local field strength.
For the ground state of an electrorheological solid of hard spheres, we
identified a novel structure transformation from the BCT to the face-centered
cubic (FCC) lattices by changing the uniaxial lattice constant c under the hard
sphere constraint. In contrast to the previous results, the local field
exhibits a non-monotonic transition from BCT to FCC. As c increases from the
BCT ground state, the local field initially decreases rapidly towards the
isotropic value at the body-centered cubic lattice, decreases further, reaching
a minimum value and increases, passing through the isotropic value again at an
intermediate lattice, reaches a maximum value and finally decreases to the FCC
value. An experimental realization of the structure transformation is
suggested. Moreover, the change in the local field can lead to a generalized
Clausius-Mossotti equation for the BCT lattices.Comment: Submitted to Phys. Rev.
Ejection of a Low Mass Star in a Young Stellar System in Taurus
We present the analysis of high angular resolution VLA radio observations,
made at eleven epochs over the last 20 years, of the multiple system T Tauri.
One of the sources (Sb) in the system has moved at moderate speed (5-10 km/s),
on an apparently elliptical orbit during the first 15 years of observations,
but after a close (< 2 AU) encounter with the source Sa, it appears to have
accelerated westward to about 20 km/s in the last few years. Such a dramatic
orbital change most probably indicates that Sb has just suffered an ejection -
which would be the first such event ever detected. Whether Sb will ultimately
stay on a highly elliptical bound orbit, or whether it will leave the system
altogether will be known with about five more years of observations.Comment: 4 pages, accepter in ApJ Letter
Density-matrix theory of the optical dynamics and transport in quantum cascade structures: The role of coherence
The impact of coherence on the nonlinear optical response and stationary
transport is studied in quantum cascade laser structures. Nonequilibrium
effects such as pump-probe signals, the spatio-temporally resolved electron
density evolution, and the subband population dynamics (Rabi flopping) as well
as the stationary current characteristics are investigated within a microscopic
density-matrix approach. Focusing on the stationary current and the recently
observed gain oscillations, it is found that the inclusion of coherence leads
to observable coherent effects in opposite parameter regimes regarding the
relation between the level broadening and the tunnel coupling across the main
injection barrier. This shows that coherence plays a complementary role in
stationary transport and nonlinear optical dynamics in the sense that it leads
to measurable effects in opposite regimes. For this reason, a fully coherent
consideration of such nonequilibrium structures is necessary to describe the
combined optical and transport propertiesComment: 14 pages, 11 figures; final versio
Effects of geometric anisotropy on local field distribution: Ewald-Kornfeld formulation
We have applied the Ewald-Kornfeld formulation to a tetragonal lattice of
point dipoles, in an attempt to examine the effects of geometric anisotropy on
the local field distribution. The various problems encountered in the
computation of the conditionally convergent summation of the near field are
addressed and the methods of overcoming them are discussed. The results show
that the geometric anisotropy has a significant impact on the local field
distribution. The change in the local field can lead to a generalized
Clausius-Mossotti equation for the anisotropic case.Comment: Accepted for publications, Journal of Physics: Condensed Matte
Spectral Signatures of the Diffusional Anomaly in Water
Analysis of power spectrum profiles for various tagged particle quantities in
bulk SPC/E water is used to demonstrate that variations in mobility associated
with the diffusional anomaly are mirrored in the exponent of the \onebyf\
region. Monitoring of \onebyf behaviour is shown to be a simple and direct
method for linking phenomena on three distinctive length and time scales: the
local molecular environment, hydrogen bond network reorganisations and the
diffusivity. The results indicate that experimental studies of supercooled
water to probe the density dependence of spectral features, or
equivalent stretched exponential behaviour in time-correlation functions, will
be of interest.Comment: 5 Pages, 4 Figure
Anomalous density dependence of static friction in sand
We measured experimentally the static friction force on the surface of
a glass rod immersed in dry sand. We observed that is extremely sensitive
to the closeness of packing of grains. A linear increase of the grain-density
yields to an exponentially increasing friction force. We also report on a novel
periodicity of during gradual pulling out of the rod. Our observations
demonstrate the central role of grain bridges and arches in the macroscopic
properties of granular packings.Comment: plain tex, 6 pages, to appear in Phys.Rev.
Symmetry-Breaking Motility
Locomotion of bacteria by actin polymerization, and in vitro motion of
spherical beads coated with a protein catalyzing polymerization, are examples
of active motility. Starting from a simple model of forces locally normal to
the surface of a bead, we construct a phenomenological equation for its motion.
The singularities at a continuous transition between moving and stationary
beads are shown to be related to the symmetries of its shape. Universal
features of the phase behavior are calculated analytically and confirmed by
simulations. Fluctuations in velocity are shown to be generically
non-Maxwellian and correlated to the shape of the bead.Comment: 4 pages, 2 figures, REVTeX; formatting of references correcte
- …