First-principles Calculations of Engineered Surface Spin Structures

The engineered spin structures recently built and measured in scanning tunneling microscope experiments are calculated using density functional theory. By determining the precise local structure around the surface impurities, we find the Mn atoms can form molecular structures with the binding surface, behaving like surface molecular magnets. The spin structures are confirmed to be antiferromagnetic, and the exchange couplings are calculated within 8% of the experimental values simply by collinear-spin GGA+U calculations. We can also explain why the exchange couplings significantly change with different impurity binding sites from the determined local structure. The bond polarity is studied by calculating the atomic charges with and without the Mn adatoms

Congress and National Strategy

While the author suggests that he is not sure what is gnawing at the innards of Congress as regards national security, he does indicate two dominant and interrelated themes

Temperature measurements behind reflected shock waves in air

A radiometric method for the measurement of gas temperature in self-absorbing gases has been applied in the study of shock tube generated flows. This method involves making two absolute intensity measurements at identical wavelengths, but for two different pathlengths in the same gas sample. Experimental results are presented for reflected shock waves in air at conditions corresponding to incident shock velocities from 7 to 10 km/s and an initial driven tube pressure of 1 torr. These results indicate that, with this technique, temperature measurements with an accuracy of + or - 5 percent can be carried out. The results also suggest certain facility related problems

Ideal strengths and bonding properties of PuO2 under tension

We perform a first-principles computational tensile test on PuO$_{2}$ based on density-functional theory within local density approximation (LDA)+\emph{U} formalism to investigate its structural, mechanical, magnetic, and intrinsic bonding properties in the four representative directions: [001], [100], [110], and [111]. The stress-strain relations show that the ideal tensile strengths in the four directions are 81.2, 80.5, 28.3, and 16.8 GPa at strains of 0.36, 0.36, 0.22, and 0.18, respectively. The [001] and [100] directions are prominently stronger than other two directions since that more Pu$-$O bonds participate in the pulling process. Through charge and density of states analysis along the [001] direction, we find that the strong mixed ionic/covalent character of Pu$-$O bond is weakened by tensile strain and PuO$_{2}$ will exhibit an insulator-to-metal transition after tensile stress exceeds about 79 GPa.Comment: 11 pages, 6 figure

Feasibility of using neutron radiography to inspect the Space Shuttle solid rocket booster aft skirt, forward skirt and frustum. Part 1: Summary report

The space shuttle's solid rocket boosters (SRB) include components made primarily of aluminum that are parachuted back for retrieval from the ocean and refurbished for repeated usage. Nondestructive inspection methods used on these aging parts to reduce the risk of unforeseen problems include x-ray, ultrasonics, and eddy current. Neutron radiography tests on segments of an SRB component show that entrapped moisture and naturally occurring aluminum corrosion can be revealed by neutron radiography even if present in only small amounts. Voids in sealant can also be evaluated. Three alternatives are suggested to follow-up this study: (1) take an SRB component to an existing neutron radiography system; (2) take an existing mobile neutron radiography system to the NASA site; or (3) plan a dedicated system custom designed for NASA applications