25,976 research outputs found
Development of the Trident 1 aerodynamic saike mechanism
The Aerospike drag reduction mechanism was designed and developed for use on the Trident I submarine launched ballistic missile. This mechanism encounters a unique combination of environments necessitating unique design solutions to ensure satisfactory operation over its design life. The development of the Aerospike is reviewed emphasizing the unique and interesting problems encountered and their solutions
Molecular observation of contour-length fluctuations limiting topological confinement in polymer melts
In order to study the mechanisms limiting the topological chain confinement in polymer melts, we have performed neutron-spin-echo investigations of the single-chain dynamic-structure factor from polyethylene melts over a large range of chain lengths. While at high molecular weight the reptation model is corroborated, a systematic loosening of the confinement with decreasing chain length is found. The dynamic-structure factors are quantitatively described by the effect of contour-length fluctuations on the confining tube, establishing this mechanism on a molecular level in space and time
Word contexts enhance the neural representation of individual letters in early visual cortex
Visual context facilitates perception, but how this is neurally implemented remains unclear. One example of contextual facilitation is found in reading, where letters are more easily identified when embedded in a word. Bottom-up models explain this word advantage as a post-perceptual decision bias, while top-down models propose that word contexts enhance perception itself. Here, we arbitrate between these accounts by presenting words and nonwords and probing the representational fidelity of individual letters using functional magnetic resonance imaging. In line with top-down models, we find that word contexts enhance letter representations in early visual cortex. Moreover, we observe increased coupling between letter information in visual cortex and brain activity in key areas of the reading network, suggesting these areas may be the source of the enhancement. Our results provide evidence for top-down representational enhancement in word recognition, demonstrating that word contexts can modulate perceptual processing already at the earliest visual regions
Next Generation Higgs Bosons: Theory, Constraints and Discovery Prospects at the Large Hadron Collider
Particle physics model building within the context of string theory suggests
that further copies of the Higgs boson sector may be expected. Concerns
regarding tree-level flavor changing neutral currents are easiest to allay if
little or no couplings of next generation Higgs bosons are allowed to Standard
Model fermions. We detail the resulting general Higgs potential and mass
spectroscopy in both a Standard Model extension and a supersymmetric extension.
We present the important experimental constraints from meson-meson mixing,
loop-induced decays and LEP2 direct production limits. We
investigate the energy range of valid perturbation theory of these ideas. In
the supersymmetric context we present a class of examples that marginally aids
the fine-tuning problem for parameter space where the lightest Higgs boson mass
is greater than the Standard Model limit of 114 GeV. Finally, we study collider
physics signatures generic to next generation Higgs bosons, with special
emphasis on signal events, and describe the capability of
discovery at the Large Hadron Collider.Comment: 43 pages, 12 figures; v3: minor corrections, published in Physical
Review
Lieb-Mattis ferrimagnetism in diluted magnetic semiconductors
We show the possibility of long-range ferrimagnetic ordering with a
saturation magnetisation of the order of 1 Bohr magneton per spin for
arbitrarily low concentration of magnetic impurities in semiconductors,
provided that the impurities form a superstructure satisfying the conditions of
the Lieb-Mattis theorem. Explicit examples of such superstructures are given
for the wurtzite lattice, and the temperature of ferrimagnetic transition is
estimated from a high-temperature expansion. Exact diagonalization studies show
that small fragments of the structure exhibit enhanced magnetic response and
isotropic superparamagnetism at low temperatures. A quantum transition in a
high magnetic field is considered and similar superstructures in cubic
semiconductors are discussed as well.Comment: 6 pages,4 figure
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