52,661 research outputs found
Hoop/column antenna deployment mechanism overview
The hoop/column antenna program is directed toward the development of a cost effective, large area, self deploying reflector antenna system. Large surface area antenna systems are required in future space missions involving improved land communications, Earth resources observation, and the study of intergalactic energy sources. The hoop/column antenna is a concept where a large antenna system can be packaged within the Space Transportation System (Shuttle) payload bay, launched into Earth orbit where it is released either for deployment as an Earth observation or communications antenna, or boosted into deep space as an intergalactic energy probe. Various mechanisms and support structures are described that are required to deploy the hoop, which is used to support the antenna reflective surface, and the column that is used to position the antenna feeds and the reflector. It also describes a proof-of-concept model (15 meters in diameter) that is currently being ground tested to determine the adequacy of the deployment mechanisms
Jahn-Teller instability in C6H6+ and C6H6- revisited
The benzene cation (C6H6+) has a doublet (e_{1g}) ground state in hexagonal
ring (D_{6h}) geometry. Therefore a Jahn-Teller (JT) distortion will lower the
energy. The present theoretical study yields a model Huckel-type Hamiltonian
that includes the JT coupling of the e_{1g} electronic ground state with the
two e_{2g} vibrational modes: in-plane ring-bending and C-C bond-stretching. We
obtain the JT couplings from density functional theory (DFT), which gives a JT
energy lowering of 970 cm^{-1} in agreement with previous quantum chemistry
calculations. We find a non-adiabatic solution for vibrational spectra and
predict frequencies shifts of both the benzene cation and anion, and give a
reinterpretation of the available experimental data.Comment: 6 pages, 3 figure
Ab initio mass tensor molecular dynamics
Mass tensor molecular dynamics was first introduced by Bennett [J. Comput.
Phys. 19, 267 (1975)] for efficient sampling of phase space through the use of
generalized atomic masses. Here, we show how to apply this method to ab initio
molecular dynamics simulations with minimal computational overhead. Test
calculations on liquid water show a threefold reduction in computational effort
without making the fixed geometry approximation. We also present a simple
recipe for estimating the optimal atomic masses using only the first
derivatives of the potential energy.Comment: 19 pages, 5 figure
Maximally Symmetric Spin-Two Bitensors on and
The transverse traceless spin-two tensor harmonics on and may be
denoted by . The index labels the (degenerate) eigenvalues
of the Laplacian and the other indices. We compute the bitensor
where are distinct
points on a sphere or hyperboloid of unit radius. These quantities may be used
to find the correlation function of a stochastic background of gravitational
waves in spatially open or closed Friedman-Robertson-Walker cosmologies.Comment: 12 pages, RevTeX, uuencoded compressed .tex file, minor typos
correcte
Quantifying Strategic Choice Along the Vertical Coordination Continuum
The qualitative and quantitative results of a study undertaken to test a decision framework firms might consider in choosing a vertical coordination strategy are presented. The posited five-step decision making process tested that a change in coordination strategy would occur if and only if a "yes" decision was made at each step. The results reported as case-based frequencies and as a discriminate analysis function provide strong support for the study's research propositions. The ability of an alternative to reduce the costliness of a coordination error and the acceptability of the risk/return tradeoff were critical to the willingness of a sample of producers to change coordination strategy. Implementability was significant, but not to the same extent as costliness of a coordination error or acceptability of the risk/return tradeoff.vertical coordination, vertical coordination continuum, discriminate analysis, willingness to change, unwillingness to change, coordination error, programmability, implementability, risk/return tradeoff, Industrial Organization,
Chi-square test on candidate events from CW signal coherent searches
In a blind search for continuous gravitational wave signals scanning a wide
frequency band one looks for candidate events with significantly large values
of the detection statistic. Unfortunately, a noise line in the data may also
produce a moderately large detection statistic.
In this paper, we describe how we can distinguish between noise line events
and actual continuous wave (CW) signals, based on the shape of the detection
statistic as a function of the signal's frequency. We will analyze the case of
a particular detection statistic, the F statistic, proposed by Jaranowski,
Krolak, and Schutz.
We will show that for a broad-band 10 hour search, with a false dismissal
rate smaller than 1e-6, our method rejects about 70 % of the large candidate
events found in a typical data set from the second science run of the Hanford
LIGO interferometer.Comment: proceedings of GWDAW8, 2003 conference, 12pages, 6 figure
Cosmic string loops and large-scale structure
We investigate the contribution made by small loops from a cosmic string
network as seeds for large-scale structure formation. We show that cosmic
string loops are highly correlated with the long-string network on large scales
and therefore contribute significantly to the power spectrum of density
perturbations if the average loop lifetime is comparable to or above one Hubble
time. This effect further improves the large-scale bias problem previously
identified in earlier studies of cosmic string models.Comment: 5 pages, 5 figure
Two-Pulse Propagation in a Partially Phase-Coherent Medium
We analyze the effects of partial coherence of ground state preparation on
two-pulse propagation in a three-level medium, in contrast to
previous treastments that have considered the cases of media whose ground
states are characterized by probabilities (level populations) or by probability
amplitudes (coherent pure states). We present analytic solutions of the
Maxwell-Bloch equations, and we extend our analysis with numerical solutions to
the same equations. We interpret these solutions in the bright/dark dressed
state basis, and show that they describe a population transfer between the
bright and dark state. For mixed-state media with partial ground
state phase coherence the dark state can never be fully populated. This has
implications for phase-coherent effects such as pulse matching, coherent
population trapping, and electromagnetically induced transparency (EIT). We
show that for partially phase-coherent three-level media, self induced
transparency (SIT) dominates EIT and our results suggest a corresponding
three-level area theorem.Comment: 29 pages, 12 figures. Submitted to Phys. Rev.
Comparison of Dissipative Particle Dynamics and Langevin thermostats for out-of-equilibrium simulations of polymeric systems
In this work we compare and characterize the behavior of Langevin and
Dissipative Particle Dynamics (DPD) thermostats in a broad range of
non-equilibrium simulations of polymeric systems. Polymer brushes in relative
sliding motion, polymeric liquids in Poiseuille and Couette flows, and
brush-melt interfaces are used as model systems to analyze the efficiency and
limitations of different Langevin and DPD thermostat implementations. Widely
used coarse-grained bead-spring models under good and poor solvent conditions
are employed to assess the effects of the thermostats. We considered
equilibrium, transient, and steady state examples for testing the ability of
the thermostats to maintain constant temperature and to reproduce the
underlying physical phenomena in non-equilibrium situations. The common
practice of switching-off the Langevin thermostat in the flow direction is also
critically revisited. The efficiency of different weight functions for the DPD
thermostat is quantitatively analyzed as a function of the solvent quality and
the non-equilibrium situation.Comment: 12 pages, introduction improved, references added, to appear in Phys.
Rev.
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