First-principles study of the Young's modulus of Si <001> nanowires

We report the results of first-principles density functional theory calculations of the Young's modulus and other mechanical properties of hydrogen-passivated Si nanowires. The nanowires are taken to have predominantly {100} surfaces, with small {110} facets. The Young's modulus, the equilibrium length and the residual stress of a series of prismatic wires are found to have a size dependence that scales like the surface area to volume ratio for all but the smallest wires. We analyze the physical origin of the size dependence, and compare the results to two existing models.Comment: 5 pages, 3 figure

Coupling of Length Scales and Atomistic Simulation of MEMS Resonators

We present simulations of the dynamic and temperature dependent behavior of Micro-Electro-Mechanical Systems (MEMS) by utilizing recently developed parallel codes which enable a coupling of length scales. The novel techniques used in this simulation accurately model the behavior of the mechanical components of MEMS down to the atomic scale. We study the vibrational behavior of one class of MEMS devices: micron-scale resonators made of silicon and quartz. The algorithmic and computational avenue applied here represents a significant departure from the usual finite element approach based on continuum elastic theory. The approach is to use an atomistic simulation in regions of significantly anharmonic forces and large surface area to volume ratios or where internal friction due to defects is anticipated. Peripheral regions of MEMS which are well-described by continuum elastic theory are simulated using finite elements for efficiency. Thus, in central regions of the device, the motion of millions of individual atoms is simulated, while the relatively large peripheral regions are modeled with finite elements. The two techniques run concurrently and mesh seamlessly, passing information back and forth. This coupling of length scales gives a natural domain decomposition, so that the code runs on multiprocessor workstations and supercomputers. We present novel simulations of the vibrational behavior of micron-scale silicon and quartz oscillators. Our results are contrasted with the predictions of continuum elastic theory as a function of size, and the failure of the continuum techniques is clear in the limit of small sizes. We also extract the Q value for the resonators and study the corresponding dissipative processes.Comment: 10 pages, 10 figures, to be published in the proceedings of DTM '99; LaTeX with spie.sty, bibtex with spiebib.bst and psfi

First-principles calculation of mechanical properties of Si <001> nanowires and comparison to nanomechanical theory

We report the results of first-principles density functional theory calculations of the Young's modulus and other mechanical properties of hydrogen-passivated Si nanowires. The nanowires are taken to have predominantly {100} surfaces, with small {110} facets according to the Wulff shape. The Young's modulus, the equilibrium length and the constrained residual stress of a series of prismatic beams of differing sizes are found to have size dependences that scale like the surface area to volume ratio for all but the smallest beam. The results are compared with a continuum model and the results of classical atomistic calculations based on an empirical potential. We attribute the size dependence to specific physical structures and interactions. In particular, the hydrogen interactions on the surface and the charge density variations within the beam are quantified and used both to parameterize the continuum model and to account for the discrepancies between the two models and the first-principles results.Comment: 14 pages, 10 figure

Structures and transitions in bcc tungsten grain boundaries and their role in the absorption of point defects

We use atomistic simulations to investigate grain boundary (GB) phase transitions in el- emental body-centered cubic (bcc) metal tungsten. Motivated by recent modeling study of grain boundary phase transitions in [100] symmetric tilt boundaries in face-centered cu- bic (fcc) copper, we perform a systematic investigation of [100] and [110] symmetric tilt high-angle and low-angle boundaries in bcc tungsten. The structures of these boundaries have been investigated previously by atomistic simulations in several different bcc metals including tungsten using the the {\gamma}-surface method, which has limitations. In this work we use a recently developed computational tool based on the USPEX structure prediction code to perform an evolutionary grand canonical search of GB structure at 0 K. For high-angle [100] tilt boundaries the ground states generated by the evolutionary algorithm agree with the predictions of the {\gamma}-surface method. For the [110] tilt boundaries, the search predicts novel high-density low-energy grain boundary structures and multiple grain boundary phases within the entire misorientation range. Molecular dynamics simulation demonstrate that the new structures are more stable at high temperature. We observe first-order grain boundary phase transitions and investigate how the structural multiplicity affects the mechanisms of the point defect absorption. Specifically, we demonstrate a two-step nucleation process, when initially the point defects are absorbed through a formation of a metastable GB structure with higher density, followed by a transformation of this structure into a GB interstitial loop or a different GB phase.Comment: 40 pages, 19 figure

Apollo Exploration Shelter System

This paper presents two unique personnel shelter systems. When functioning either alone or with MOLAB, they will provide the capability for maintaining 2 or 3 astronauts on the lunar surface for a duration of 30 to 44 Earth days with a stay-time growth potential of 90 days. These shelters offer the United States Lunar Exploration Program the operational flexibility and number of man-days on the Moon required to accomplish a wide range of necessary exploration and scientific missions, including a capability to maintain and repair the LEM, MOLAB, and the shelters themselves. An additional feature of the system is that a Lunar Roving Vehicle or a Lunar Observatory as well as a Shelter can be transported on the LEM truck in a single flight to the lunar surface by the Saturn V transportation system. Further, the shelter system concept optimizes the systems approach to lurtar exploration in terms of timeliness, operational flexibility, growth potential, and return on the Apollo investment

The potential for circular dichroism as an additional facile and sensitive method of monitoring low-molecular-weight heparins and heparinoids

The ultraviolet circular dichroism (CD) spectra of commercial low-molecular-weight heparins, heparinoids and other anticoagulant preparations have been recorded between 180 and 260 nm. Principal component analysis of the spectra allowed their differentiation into a number of groups related to the means of their production reflecting the structural changes introduced by each process. The findings suggest that CD provides a complementary technique for the rapid analysis of heparin preparations

Utilizing the Innovative Leadership Behavior Inventory and Relationship Marketing as Critical Elements for Teaching/Learning Entrepreneurial Leadership (EL)

The focus of this paper will be on utilizing the five-factor Leader Behavior Inventory (LBI) as the structure, and various teaching or learning pedagogy and related processes and relationships as the intervening variables in order to help entrepreneurs assess then enhance their potential leadership behavior. In turn, this should foster the decision process necessary to accomplish enterprise building or organizational development thus enhancing the cycle time for critical change. Should the LBI and associated assessment tools and processes indicate such, the best practice strategies may involve bringing in professional management, slowing the growth of the enterprise to allow for leadership development, or an appropriate exit strategy

The Josephson Effect in Single Spin Superconductors

The Josephson Effect provides a primary signature of single spin superconductivity (SSS), the as yet unobserved superconducting state which was proposed recently as a low temperature phase of half-metallic antiferromagnets. These materials are insulating in the spin-down channel but are metallic in the spin-up channel. The SSS state is characterized by a unique form of p-wave pairing within a single spin channel. We develop the theory of a rich variety of Josephson effects that arise due to the form of the SSS order parameter. Tunneling is allowed at a SSS-SSS' junction but of course depends on the relative orientation of their order parameters. No current flows between an SSS and an s-wave BCS system due to their orthogonal symmetries, which potentially can be used to distinguish SSS from other superconducting states. Single spin superconductors also offer a means to probe other materials, where tunneling is a litmus test for any form of ``triplet'' order parameter.Comment: 4 pages, RevTeX, 2 PostScript figures included, to appear in J. Phys. and Chem. of Solid