2,191 research outputs found
Reconstruction, rumpling, and Dirac states at the (001) surface of a topological crystalline insulator Pb1-xSnxSe
Equilibrium atomic configuration and electronic structure of the (001)
surface of IV-VI semiconductors PbTe, PbSe, SnTe and SnSe, is studied using the
density functional theory (DFT) methods. At surfaces of all those compounds,
the displacements of ions from their perfect lattice sites reveal two features
characteristic of the rock salt crystals. First, the ionic displacements occur
only along the direction perpendicular to the surface, and they exhibit the
rumpling effect, i.e., the vertical shifts of cations and anions differ.
Second, the interlayer spacing of the first few monolayers at the surface
oscillates. Our results are in good agreement with the previous X-ray
experimental data and theoretical results where available. They also are
consistent with the presence of two {110} mirror planes at the (001) surface of
the rock salt. One the other hand, experiments preformed for the topological
PbSn Se alloy indicate breaking of the mirror symmetry due to a
large 0.3 {\AA} relative displacement of the cation and anion sublattices at
the surface, which induces the opening of the gap of the Dirac cones. Our
results for PbSnSe including the simulated STM images, are in
contradiction with these findings, since surface reconstructions with broken
symmetry are never the ground state configurations. The impact of the
theoretically determined surface configurations and of the chemical disorder on
the surface states is analyzed.Comment: 9 pages, 11 figure
The Effect of Diethylstilbestrol on the Digestibility of Dry Matter and Nitrogen and on Nitrogen Retention in Lambs
Although the usefulness of orally administered diethylstilbestrol (stilbestrol) in increasing weight gains and feed efficiency of fattening cattle has been established, little is known concerning the mechanism by which it exerts its beneficial effect. Two possible modes of action may exist. First, the stilbestrol may have some effect on the rumen microorganisms which might cause an increased digestion of feed in the rumen. Brooks et al. (1954) have shown that stilbestrol increased the digestibility of cellulose in the artificial rumen. They also obtained increased cellulose and protein digestion in sheep when stilbestrol was fed. However the levels fed the sheep were considerably above the mg. per lamb per day reported to be effective with lambs (Hale et al. 1955). Sykes et al. (1956) reported an increase in crude fiber digestibility and a decrease in protein digestibility with lactating cows when stilbestrol was fed. Digestibility of the dry matter of the ration tended to be improved but the differences were not statistically significant. Erwin et al. (1956) reported stilbestroI had no effect on digestibility of dry matter, crude fiber, crude protein or ether extract with steers. Secondly, the orally fed stilbestrol may exert some action on the metabolism of the animal\u27s tissue which is thought to occur when the stilbestrol is implanted (Clegg and Cole, 19 54). It has been shown that implanted stilbestrol increased nitrogen retention but had no effect on ration digestibility (Jordan 1953: Whitehair et al. 1953). Bell et al. (1955) found that orally fed stilbestrol increased nitrogen retention in lambs. Presumably this action is brought about by the absorbed stilbestrol acting similar to that on the implanted stilbestrol. The objectives of this report were to study the effects of different levels of stilbestrol upon the digestibility of dry matter and crude protein and on nitrogen retention with wether lambs
An Overview of Propulsion Concept Studies and Risk Reduction Activities for Robotic Lunar Landers
In support of designing robotic lunar lander concepts, the propulsion team at NASA Marshall Space Flight Center (MSFC) and the Johns Hopkins University Applied Physics Laboratory (APL), with participation from industry, conducted a series of trade studies on propulsion concepts with an emphasis on light-weight, advanced technology components. The results suggest a high-pressure propulsion system may offer some benefits in weight savings and system packaging. As part of the propulsion system, a solid rocket motor was selected to provide a large impulse to reduce the spacecraft s velocity prior to the lunar descent. In parallel to this study effort, the team also began technology risk reduction testing on a high thrust-to-weight descent thruster and a high-pressure regulator. A series of hot-fire tests was completed on the descent thruster in vacuum conditions at NASA White Sands Test Facility (WSTF) in New Mexico in 2009. Preparations for a hot-fire test series on the attitude control thruster at WSTF and for pressure regulator testing are now underway. This paper will provide an overview of the concept trade study results along with insight into the risk mitigation activities conducted to date
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