Tunnel injection into GaAs/AlGaAs long period superlattices

Electron injection through a tunnel barrier into a long period GaAs/AlGaAs superlattice is investigated. Seven negative differential resistance (NDR) regions are observed, resulting from resonant tunneling into the first well of the superlattice. Their positions can be quantitatively accounted for by considering the distribution of the electric field in the depletion region and the tunnel barrier. Upon application of a magnetic field parallel to the layers, the NDR's are shifted and weakened, which can be explained in terms of conservation of energy and canonical momentum. Furthermore, optical phonon generated conductance oscillations are observed although the depletion region is punctuated by the superlattice structure.     

Three-dimensional magnetic field enhancement in a liner implosionsystem

The magnetic field enhancement is calculated for a magnetically imploded liner system that has flux excluding radial vanes. For the thinnest vane tested the field is found to concentrate at the vanes to a maximum value of almost three times its ambient value with a corresponding temperature increase well above the melting point. These values are calculated using the three-dimensional magnetohydrodgnamic code, MACH3. Calculations are performed for three vane thicknesses, and the vane movement is estimated. A bound is established on the design specification of the vanes based the disruption of current delivery to the liner due to the movement of the vanes     

Cellulose structure and biosynthesis: What is in store for the 21st century?

This article briefly summarizes historical developments in fundamental research related to the structure and biosynthesis of cellulose. Major advances concerning the structure of cellulose include the discovery of a new suballomorph of cellulose I, the lattice imaging of glucan chains showing no fringe micelle structure, parallel chain orientation in cellulose I, and the discovery of nematic ordered cellulose. Major advances in biosynthesis include the discovery of the terminal synthesizing complex, the isolation and purification of cellulose synthase, the in vitro synthesis of cellulose I, and synthetic cellulose assembly. This article focuses on recent advances in molecular biology with cellulose, including the cloning and sequencing of cellulose synthase genes from bacteria, cyanobacteria, and vascular plants; proof of the terminal synthesizing complex as the site of the catalytic subunit of cellulose synthase; cellulose and callose synthase expression during growth and development; and phylogenetic aspects of cellulose synthase evolution. This article concludes with thoughts about future uses for the accumulating genetic information on cellulose biosynthesis for textiles and forest products and discusses possibilities of new global resources for cellulose production. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 487–495, 2004     

A molecular mechanics study of neutral molecule complexation with crown ethers

Molecular mechanics methods are applied to the study of neutral molecule complexation with crown ethers. Protocols for the development of parameters necessary to describe these molecule-molecule interactions are presented. Application of these methods to the study of 1:1 and 2:1 (guest:host) complexation between acetonitrile (1), nitromethane (2), malononitrile (3) and dimethylsulfone (4) reveals positive cooperativity in formation of the 2:1 complexes in isolation.