1,983 research outputs found
Random polarization dynamics in a resonant optical medium
Random optical-pulse polarization switching along an active optical medium in
the -configuration with spatially disordered occupation numbers of its
lower energy sub-level pair is described using the idealized integrable
Maxwell-Bloch model. Analytical results describing the light
polarization-switching statistics for the single self-induced transparency
pulse are compared with statistics obtained from direct Monte-Carlo numerical
simulations.Comment: 7 pages, 3 figure
Spherical Orbifolds for Cosmic Topology
Harmonic analysis is a tool to infer cosmic topology from the measured
astrophysical cosmic microwave background CMB radiation. For overall positive
curvature, Platonic spherical manifolds are candidates for this analysis. We
combine the specific point symmetry of the Platonic manifolds with their deck
transformations. This analysis in topology leads from manifolds to orbifolds.
We discuss the deck transformations of the orbifolds and give eigenmodes for
the harmonic analysis as linear combinations of Wigner polynomials on the
3-sphere. These provide new tools for detecting cosmic topology from the CMB
radiation.Comment: 17 pages, 9 figures. arXiv admin note: substantial text overlap with
arXiv:1011.427
Statistical Mobility of Multicellular Colonies of Flagellated Swimming Cells
We study the stochastic hydrodynamics of colonies of flagellated swimming
cells, typified by multicellular choanoflagellates, which can form both rosette
and chainlike shapes. The objective is to link cell-scale dynamics to
colony-scale dynamics for various colonial morphologies. Via autoregressive
stochastic models for the cycle-averaged flagellar force dynamics and
statistical models for demographic cell-to-cell variability in flagellar
properties and placement, we derive effective transport properties of the
colonies, including cell-to-cell variability. We provide the most quantitative
detail on disclike geometries to model rosettes, but also present formulas for
the dynamics of general planar colony morphologies, which includes planar
chain-like configurations
Characterization of Water Dissociation on -AlO: Theory and Experiment
The interaction of water with -alumina (i.e. -AlO
surfaces is important in a variety of applications and a useful model for the
interaction of water with environmentally abundant aluminosilicate phases.
Despite its significance, studies of water interaction with
-AlO surfaces other than the are extremely
limited. Here we characterize the interaction of water (DO) with a well
defined -AlO surface in UHV both
experimentally, using temperature programmed desorption and surface-specific
vibrational spectroscopy, and theoretically, using periodic-slab density
functional theory calculations. This combined approach makes it possible to
demonstrate that water adsorption occurs only at a single well defined surface
site (the so-called 1-4 configuration) and that at this site the barrier
between the molecularly and dissociatively adsorbed forms is very low: 0.06 eV.
A subset of OD stretch vibrations are parallel to this dissociation coordinate,
and thus would be expected to be shifted to low frequencies relative to an
uncoupled harmonic oscillator. To quantify this effect we solve the vibrational
Schr\"odinger equation along the dissociation coordinate and find fundamental
frequencies red-shifted by more than 1,500 cm. Within the context
of this model, at moderate temperatures, we further find that some fraction of
surface deuterons are likely delocalized: dissociatively and molecularly
absorbed states are no longer distinguishable.Comment: Paper: 22 pages, 9 figures , ESI: 6 pages, 1 figur
Statics and dynamics of free and hydrogen-bonded OH groups at the air/water interface
We use classical atomistic molecular dynamics simulations of two water models (SPC/E and TIP4P/2005) to investigate the orientation and reorientation dynamics of two subpopulations of OH groups belonging to water molecules at the air/water interface at 300 K: those OH groups that donate a hydrogen bond (called “bonded”) and those that do not (called “free”). Free interfacial OH groups reorient in two distinct regimes: a fast regime from 0 to 1 ps and a slow regime thereafter. Qualitatively similar behavior was reported by others for free OH groups near extended hydrophobic surfaces. In contrast, the net reorientation of bonded OH groups occurs at a rate similar to that of bulk water. This similarity in reorientation rate results from compensation of two effects: decreasing frequency of hydrogen-bond breaking/formation (i.e., hydrogen-bond exchange) and faster rotation of intact hydrogen bonds. Both changes result from the decrease in density at the air/water interface relative to the bulk. Interestingly, because of the presence of capillary waves, the slowdown of hydrogen-bond exchange is signiffcantly smaller than that reported for water near extended hydrophobic surfaces, but it is almost identical to that reported for water near small hydrophobic solutes. In this sense water at the air/water interface has characteristics of water of hydration of both small and extended hydrophobic solutes.SARA Computing and Networking Services (www.sara.nl)Nederlandse Organisatie voor Wetenschappelijk Onderzoe
Lensing Properties of Lightlike Current Carrying Cosmic Strings
The lensing properties of superconducting cosmic strings endowed with a time
dependent pulse of lightlike current are investigated. The metric outside the
core of the string belongs to the --wave class, with a deficit angle. We
study the field theoretic bosonic Witten model coupled to gravity, and we show
that the full metric (both outside and inside the core) is a Taub-Kerr-Shild
generalization of that for the static string with no current. It is shown that
the double image due to the deficit angle evolves in an unambiguous way as a
pulse of lightlike current passes between the source and the observer.
Observational consequences of this signature of the existence of cosmic strings
are briefly discussed.Comment: 21 pages, LaTeX-REVTeX, 7 figures available upon request, preprint #
DAMTP-R94/1
Asymptotic Analysis of Microtubule-Based Transport by Multiple Identical Molecular Motors
We describe a system of stochastic differential equations (SDEs) which model
the interaction between processive molecular motors, such as kinesin and
dynein, and the biomolecular cargo they tow as part of microtubule-based
intracellular transport. We show that the classical experimental environment
fits within a parameter regime which is qualitatively distinct from conditions
one expects to find in living cells. Through an asymptotic analysis of our
system of SDEs, we develop a means for applying in vitro observations of the
nonlinear response by motors to forces induced on the attached cargo to make
analytical predictions for two parameter regimes that have thus far eluded
direct experimental observation: 1) highly viscous in vivo transport and 2)
dynamics when multiple identical motors are attached to the cargo and
microtubule
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