36,509 research outputs found

    The origin of Sr segregation at La1-xSrxMnO3 surfaces

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    A uniform distribution of La and Sr in lanthanum-strontium manganites would lead to charged crystal planes, a charged surface, and arbitrarily large surface energy for a bulk crystal. This divergent energy can be eliminated by depleting the La concentration near the surface. Assuming an exponential form for segregation suggested by experiment, the total electrostatic energy is calculated, depending only upon the decay length and on an effective charge Z* associated with the La ion. It is found to be lower in energy than neutralization of the surface by changing Mn charge states, previously expected, and lower than simply readjusting the La concentration in the surface plane. The actual decay length obtained by minimizing this electrostatic energy is shorter than that observed. The extension of this mechanism to segregation near the surface in other systems is discussed

    The Abrikosov Flux Lattice in Planar Crystals of YBaCuO

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    Anisotropic London Theory is used to predict the Abriskosv flux lattice in planar crystals of YBaCuO. By taking into account the orientation of the flux lattice as a function of applied field it is shown that the vortex chain state is observed in Bitter pattern experiments.Comment: 17 pages, Late

    Self-consistent energy balance simulations of hole dynamics in SiGe/Si THz quantum cascade structures

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    Analysis of hole transport in cascaded p-Si/SiGe quantum well structures is performed using self-consistent rate equations simulations. The hole subband structure is calculated using the 6×6 k·p model, and then used to find carrier relaxation rates due to the alloy disorder, acoustic, and optical phonon scattering, as well as hole-hole scattering. The simulation accounts for the in-plane k-space anisotropy of both the hole subband structure and the scattering rates. Results are presented for prototype THz Si/SiGe quantum cascade structures. © 2004 American Institute of Physic

    Experimental Methods and the Welfare Evaluation of Policy Lotteries

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    Policies impose lotteries of outcomes on individuals, since we never know exactly what the effects of the policy will be. In order to evaluate alternative policies, we therefore need to make some assumptions about individual preferences, even before social welfare functions are applied. Instead of making a priori assumptions about those preferences that are likely to be wrong, there are two broad ways in which experimental methods are used to evaluate policy. One is to use experiments to estimate individual preferences, valuations and beliefs, and use those estimates as priors in the evaluation of policy. The other approach is to undertake deliberate randomization, or exploit accidental or natural randomization, to infer the effects of policy. The strengths and weaknesses of these approaches are reviewed, and their complementarities identified.

    Advanced refractory alloy corrosion loop program Quarterly progress report, 15 Apr. - 15 Jul. 1967

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    Fabrication and evaluation of Rankine system refractory alloy corrosion test loo

    Advanced refractory alloy corrosion loop program Quarterly progress report no. 4, Jan. 15 - Apr. 15, 1966

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    Refractory alloy materials procurement and evaluation for potassium corrosion loop progra