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This Week at Physics

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Events on Thursday, November 6th, 2014

R. G. Herb Condensed Matter Seminar
Accurate and efficient simulation of donors in silicon
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: John King Gamble, Sandia National Laboratories
Abstract: For the past sixty years, researchers have studied the electronic structure of donors in silicon using the effective mass approximation, where electronic states are restricted to the vicinity of silicon's six conduction band minima. Despite including central cell corrections and valley-orbit coupling, effective mass theories to date are typically regarded as phenomenological tools, while more computationally-intensive atomistic simulations are more trustworthy.

Here, we present a fully internally consistent effective mass theory that includes
valley-orbit coupling and relies upon only a few approximations. Inspired by recent density functional theory calculations, we include a tetrahedral central cell correction, variationally matching experimentally measured energy levels of phosphorous donors in silicon. When imposing internal consistency, we find both the form of the central cell and the Bloch functions are critically important to obtaining agreement with experiment.

Within this new effective mass framework, we obtain quantitative agreement with the NEMO 3D tight-binding code when calculating the tunnel coupling energy between two phosphorous donors in silicon, a critical quantity for donor-based quantum information processing. We then use our framework, which is several orders of magnitude faster than comparable atomistic simulations, to exhaustively enumerate tunnel coupling over a ~30 nm cube of donor placements, about 1.3 million distinct placements. This high-throughput approach enables the identifications of regions where the tunnel coupling shows little variation among nearby donor positions with high probability, suggesting the feasibility of realistic devices with regular, controllable properties.

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
Host: Mark Friesen
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Astronomy Colloquium
New Windows into Supernova Explosions and Progenitors
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Jennifer Hoffman, UW Milwaukee
Supernovae of all types exhibit time-dependent spectropolarimetric signatures produced primarily by electron scattering. These reveal the presence of aspherical phenomena such as complex velocity structures, changing illumination, and asymmetric morphologies within the ejecta or surrounding circumstellar material. The gradual thinning of the ejecta over time also allows us to probe different scattering regions as the supernova evolves. Interpreting the time variations of spectropolarimetric signatures yields unprecedentedly detailed information about supernova explosion mechanisms, the physical processes that shape the density and velocity distributions of the ejecta and circumstellar material, and the properties of the progenitor star.

I will present an overview of supernova spectropolarimetry, highlighting recent observational and computational results. This versatile technique helps us to constrain explosion mechanisms, connect SNe with their massive progenitors (as well as other high-energy transient phenomena such as GRBs), and investigate the process of stellar evolution in other galaxies.
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Graduate Introductory Seminar
WIPAC: IceCube, HAWC, Cosmic Ray Physics
Time: 4:30 pm
Place: 5280 Chamberlin Hal
Speaker: Halzen, Hanson, Karle, Vandenbroucke, Westerhoff
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