16,768 research outputs found
Mode coupling theory for the critical dynamics of dipolar ferromagnets Hyperfine Interactions
Current reversal and exclusion processes with history-dependent random walks
A class of exclusion processes in which particles perform history-dependent
random walks is introduced, stimulated by dynamic phenomena in some biological
and artificial systems. The particles locally interact with the underlying
substrate by breaking and reforming lattice bonds. We determine the
steady-state current on a ring, and find current-reversal as a function of
particle density. This phenomenon is attributed to the non-local interaction
between the walkers through their trails, which originates from strong
correlations between the dynamics of the particles and the lattice. We
rationalize our findings within an effective description in terms of
quasi-particles which we call front barriers. Our analytical results are
complemented by stochastic simulations.Comment: 5 pages, 6 figure
Stratospheric measurement requirements and satellite-borne remote sensing capabilities
The capabilities of specific NASA remote sensing systems to provide appropriate measurements of stratospheric parameters for potential user needs were assessed. This was used to evaluate the capabilities of the remote sensing systems to perform global monitoring of the stratosphere. The following conclusions were reached: (1) The performance of current remote stratospheric sensors, in some cases, compares quite well with identified measurement requirements. Their ability to measure other species has not been demonstrated. (2) None of the current, in-situ methods have the capability to satisfy the requirements for global monitoring and the temporal constraints derived from the users needs portion of the study. (3) Existing, non-remote techniques will continue to play an important role in stratospheric investigations for both corroboration of remotely collected data and in the evolutionary development of future remote sensors
Satellite-aided mobile communications limited operational test in the trucking industry
An experiment with NASA's ATS-6 satellite, that demonstrates the practicality of satellite-aided land mobile communications is described. Satellite communications equipment for the experiment was designed so that it would be no more expensive, when mass produced, than conventional two-way mobile radio equipment. It embodied the operational features and convenience of present day mobile radios. Vehicle antennas 75 cm tall and 2 cm in diameter provided good commercial quality signals to and from trucks and jeeps. Operational applicability and usage data were gathered by installing the radio equipment in five long-haul tractor-trailer trucks and two Air Force search and rescue jeeps. Channel occupancy rates are reported. Air Force personnel found the satellite radio system extremely valuable in their search and rescue mission during maneuvers and actual rescue operations. Propagation data is subjectively analyzed and over 4 hours of random data is categorized and graded as to signal quality on a second by second basis. Trends in different topographic regions are reported. An overall communications reliability of 93% was observed despite low satellite elevation angles ranging from 9 to 24 degrees
Position surveillance using one active ranging satellite and time-of-arrival of a signal from an independent satellite
Position surveillance using one active ranging/communication satellite and the time-of-arrival of signals from an independent satellite was shown to be feasible and practical. A towboat on the Mississippi River was equipped with a tone-code ranging transponder and a receiver tuned to the timing signals of the GOES satellite. A similar transponder was located at the office of the towing company. Tone-code ranging interrogations were transmitted from the General Electric Earth Station Laboratory through ATS-6 to the towboat and to the ground truth transponder office. Their automatic responses included digital transmissions of time-of-arrival measurements derived from the GOES signals. The Earth Station Laboratory determined ranges from the satellites to the towboat and computed position fixes. The ATS-6 lines-of-position were more precise than 0.1 NMi, 1 sigma, and the GOES lines-of-position were more precise than 1.6 NMi, 1 sigma. High quality voice communications were accomplished with the transponders using a nondirectional antenna on the towboat. The simple and effective surveillance technique merits further evaluation using operational maritime satellites
Ultrafast control of inelastic tunneling in a double semiconductor quantum
In a semiconductor-based double quantum well (QW) coupled to a degree of
freedom with an internal dynamics, we demonstrate that the electronic motion is
controllable within femtoseconds by applying appropriately shaped
electromagnetic pulses. In particular, we consider a pulse-driven AlxGa1-xAs
based symmetric double QW coupled to uniformly distributed or localized
vibrational modes and present analytical results for the lowest two levels.
These predictions are assessed and generalized by full-fledged numerical
simulations showing that localization and time-stabilization of the driven
electron dynamics is indeed possible under the conditions identified here, even
with a simultaneous excitations of vibrational modes.Comment: to be published in Appl.Phys.Let
Evaluation of solar cell welds by scanning acoustic microscopy
Scanning laser acoustic microscopy was used to nondestructively evaluate solar cell interconnect bonds made by resistance welding. Both copper-silver and silver-silver welds were analyzed. The bonds were produced either by a conventional parallel-gap welding technique using rectangular electrodes or new annular gap design with a circular electrode cross section. With the scanning laser acoustic microscope, it was possible to produce a real time television image which reveales the weld configuration as it relates to electrode geometry. The effect of electrode misalinement with the surface of the cell was also determined. A preliminary metallographic analysis was performed on selected welds to establish the relationship between actual size and shape of the weld area and the information available from acoustic micrographs
Nonaffine rubber elasticity for stiff polymer networks
We present a theory for the elasticity of cross-linked stiff polymer
networks. Stiff polymers, unlike their flexible counterparts, are highly
anisotropic elastic objects. Similar to mechanical beams stiff polymers easily
deform in bending, while they are much stiffer with respect to tensile forces
(``stretching''). Unlike in previous approaches, where network elasticity is
derived from the stretching mode, our theory properly accounts for the soft
bending response. A self-consistent effective medium approach is used to
calculate the macroscopic elastic moduli starting from a microscopic
characterization of the deformation field in terms of ``floppy modes'' --
low-energy bending excitations that retain a high degree of non-affinity. The
length-scale characterizing the emergent non-affinity is given by the ``fiber
length'' , defined as the scale over which the polymers remain straight.
The calculated scaling properties for the shear modulus are in excellent
agreement with the results of recent simulations obtained in two-dimensional
model networks. Furthermore, our theory can be applied to rationalize bulk
rheological data in reconstituted actin networks.Comment: 12 pages, 10 figures, revised Section II
Fluorescence Correlation Spectroscopy analysis of segmental dynamics in Actin filaments
We adapt Fluorescence Correlation spectroscopy (FCS) formalism to the studies
of the dynamics of semi-flexible polymers and derive expressions relating FCS
correlation function to the longitudinal and transverse mean square
displacements of polymer segments. We use the derived expressions to measure
the dynamics of actin filaments in two experimental situations: filaments
labeled at distinct positions and homogeneously labeled filaments. Both
approaches give consistent results and allow to measure the temporal dependence
of the segmental mean-square displacement (MSD) over almost five decades in
time, from ~0.04ms to 2s. These noninvasive measurements allow for a detailed
quantitative comparison of the experimental data to the current theories of
semi-flexible polymer dynamics. Good quantitative agreement is found between
the experimental results and theories explicitly accounting for the
hydrodynamic interactions between polymer segments
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