Strain Relaxation Mechanisms and Local Structural Changes in Si_{1-x}$Ge_{x} Alloys

In this work, we address issues pertinent to the understanding of the structural and electronic properties of Si_{1-x} Ge_{x}alloys, namely, (i) how does the lattice constant mismatch between bulk Si and bulk Ge manifests itself in the alloy system? and (ii) what are the relevant strain release mechanisms? To provide answers to these questions, we have carried out an in-depth study of the changes in the local geometric and electronic structures arising from the strain relaxation in Si_{1-x} Ge_{x} alloys using an ab initio molecular dynamics scheme. The optimized lattice constant, while exhibiting a general trend of linear dependence on the composition (Vegard's law), shows a negative deviation from Vegard's law in the vicinity of x=0.5. We delineate the mechanisms responsible for each one of the above features. We show that the radial-strain relaxation through bond stretching is responsible for the overall trend of linear dependence of the lattice constant on the composition. On the other hand, the negative deviation from Vegard's law is shown to arise from the angular-strain relaxation.Comment: 21 pages, 7 figure

A scanning transmission x-ray microscope for materials science spectromicroscopy at the advanced light source

Design and performance of a scanning transmission x-ray microscope (STXM) at the Advanced Light Source is described. This instrument makes use of a high brightness undulator beamline and extends the STXM technique to new areas of research. After 2.5 years of development it is now an operational tool for research in polymer science, environmental chemistry, and magnetic materials. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71051/2/RSINAK-69-8-2964-1.pd

Overview of the results of the organics PET Study of the cometary samples returned from comet Wild 2 by the Stardust mission

This presenation will provide an overview of the efforts and results produced by the Organics Preliminary Examination Team during their studies of the samples returned from comet Wild 2 by the Stardust spacecraft

X-ray imaging of the dynamic magnetic vortex core deformation

Magnetic platelets with a vortex configuration are attracting considerable attention. The discovery that excitation with small in-plane magnetic fields or spin polarised currents can switch the polarisation of the vortex core did not only open the possibility of using such systems in magnetic memories, but also initiated the fundamental investigation of the core switching mechanism itself. Micromagnetic models predict that the switching is mediated by a vortex-antivortex pair, nucleated in a dynamically induced vortex core deformation. In the same theoretical framework, a critical core velocity is predicted, above which switching occurs. Although these models are extensively studied and generally accepted, experimental support has been lacking until now. In this work, we have used high-resolution time-resolved X-ray microscopy to study the detailed dynamics in vortex structures. We could reveal the dynamic vortex core deformation preceding the core switching. Also, the threshold velocity could be measured, giving quantitative comparison with micromagnetic models