4,867 research outputs found

    The effects of localized damping on structural response

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    The effect of localized structural damping on the excitability of higher order normal modes of the large space telescope was investigated. A preprocessor computer program was developed to incorporate Voigt structural joint damping models in a NASTRAN finite-element dynamic model. A postprocessor computer program was developed to select critical modes for low-frequency attitude control problems and for higher frequency fine-stabilization problems. The mode selection is accomplished by ranking the flexible modes based on coefficients for rate gyro, position gyro, and optical sensors, and on image-plane motions due to sinusoidal or random power spectral density force and torque inputs

    Effect of damping on excitability of high-order normal modes

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    The effect of localized structural damping on the excitability of higher-order large space telescope spacecraft modes is investigated. A preprocessor computer program is developed to incorporate Voigt structural joint damping models in a finite-element dynamic model. A postprocessor computer program is developed to select critical modes for low-frequency attitude control problems and for higher-frequency fine-stabilization problems. The selection is accomplished by ranking the flexible modes based on coefficients for rate gyro, position gyro, and optical sensor, and on image-plane motions due to sinusoidal or random PSD force and torque inputs

    Electromagnetic Transition Strengths in Heavy Nuclei

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    We calculate reduced B(E2) and B(M1) electromagnetic transition strengths within and between K-bands in support of a recently proposed model for the structure of heavy nuclei. Previously, only spectra and a rough indication of the largest B(E2) strengths were reported. The present more detailed calculations should aid the experimental identification of the predicted 0+0^+, 1+1^+ and 2+2^+ bands and, in particular, act to confirm or refute the suggestion that the model 0+0^+ and 2+2^+ bands correspond to the well known and widespread beta and gamma bands. Furthermore they pinpoint transitions which can indicate the presence of a so far elusive 1+1^+ band by feeding relatively strongly into or out of it. Some of these transitions may already have been measured in 230^{230}Th, 232^{232}Th and 238^{238}U.Comment: 10 pages, 1 Figure, submitted to Physical Review

    Influence of surface passivation on ultrafast carrier dynamics and terahertz radiation generation in GaAs

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    The carrier dynamics of photoexcited electrons in the vicinity of the surface of (NH4)2S-passivated GaAs were studied via terahertz (THz) emission spectroscopy and optical-pump THz-probe spectroscopy. THz emission spectroscopy measurements, coupled with Monte Carlo simulations of THz emission, revealed that the surface electric field of GaAs reverses after passivation. The conductivity of photoexcited electrons was determined via optical-pump THz-probe spectroscopy, and was found to double after passivation. These experiments demonstrate that passivation significantly reduces the surface state density and surface recombination velocity of GaAs. Finally, we have demonstrated that passivation leads to an enhancement in the power radiated by photoconductive switch THz emitters, thereby showing the important influence of surface chemistry on the performance of ultrafast THz photonic devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter

    Energy transfer in nonlinear network models of proteins

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    We investigate how nonlinearity and topological disorder affect the energy relaxation of local kicks in coarse-grained network models of proteins. We find that nonlinearity promotes long-range, coherent transfer of substantial energy to specific, functional sites, while depressing transfer to generic locations. Remarkably, transfer can be mediated by the self-localization of discrete breathers at distant locations from the kick, acting as efficient energy-accumulating centers.Comment: 4 pages, 3 figure

    Beryllium in the Ultra-Lithium-Deficient,Metal-Poor Halo Dwarf, G186-26

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    The vast majority of low-metal halo dwarfs show a similar amount of Li; this has been attributed to the Li that was produced in the Big Bang. However, there are nine known halo stars with T >> 5900 K and [Fe/H] << −-1.0 that are ultra-Li-deficient. We have looked for Be in the very low metallicity star, G 186-26 at [Fe/H] = −-2.71, which is one of the ultra-Li-deficient stars. This star is also ultra-Be deficient. Relative to Be in the Li-normal stars at [Fe/H] = −-2.7, G 182-26 is down in Be by more than 0.8 dex. Of two potential causes for the Li-deficiency -- mass-transfer in a pre-blue straggler or extra rotationally-induced mixing in a star that was initially a very rapid rotator -- the absence of Be favors the blue-straggler hypothesis, but the rotation model cannot be ruled-out completely.Comment: Accepted for Ap.J. Letters 10 pages, 4 figure
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