Approximate ab initio calculation of vibrational properties of hydrogenated amorphous silicon with inner voids

We have performed an approximate ab initio calculation of vibrational properties of hydrogenated amorphous silicon (a-Si:H) using a molecular dynamics method. A 216 atom model for pure amorphous silicon (a-Si) has been employed as a starting point for our a-Si:H models with voids that were made by removing a cluster of silicon atoms out of the bulk and terminating the resulting dangling bonds with hydrogens. Our calculation shows that the presence of voids leads to localized low energy (30-50 cm^{-1}) states in the vibrational spectrum of the system. The nature and localization properties of these states are analyzed by various visualization techniques.Comment: 15 pages with 6 PS figures, to appear in PRB in December 199

Whose shoe fits best? Dubious physics and power politics in the TMD footprint controversy

Apparent design breakthroughs in short-range missile defense systems such as Theater High-Altitude Air Defense (THAAD) have prompted questions about the legality of such systems under the 1972 Antiballistic Missile (ABM) Treaty. Prominent physicists have used computer “footprint” methodology to prove that if engineered to specifications, THAAD might exceed ABM Treaty performance limits banning highly effective missile defense systems. In response, missile defense officials commissioned Sparta, Inc. to conduct secret research casting doubt on the validity of such findings. The substantial diplomatic issues at stake and the interesting rhetorical dynamics involved in this dispute warrant close scholarly analysis. Attention to the iterative relationship between the interpenetrating spheres of public argument and scientific practice in this case yields novel insight about the processes in which technical knowledge of defense systems is forged and raises fresh issues for the “closure project” in science and technology controversy studies