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This Week at Physics

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Events on Thursday, April 28th, 2011

R. G. Herb Condensed Matter Seminar
Improved Charge Collection in Nanostructured Organic Semiconductor Solar Cells
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Charles Black, Brookhaven National Lab
Abstract: High-performance organic semiconductor solar cell active layers form via a self-assembly process of phase separation of blended donor and acceptor materials. Achieving optimal device performance requires a delicate balance of trapping the blended material in a non-equilibrium configuration. I will describe our experimental efforts to confine both organic semiconductors and semiconductor blends within nanometer-scale volumes to better control material phase separation and understand the effect of geometry on the material structure, its electronic properties, and its photovoltaic performance.

As one example, confining blended poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester organic solar cell active layers within nanometer-scale cylindrical volumes more than doubles the supported photocurrent density compared to equivalent unconfined volumes of the same blend, and increases the poly(3-hexylthiophene) hole mobility in the blend by more than 500 times. Grazing incidence x-ray diffraction measurements show that the confining volume disrupts polymer ordering by reducing crystallinity and grain size, as well as changing crystal orientation.
Host: Mark Eriksson
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
A Holistic View of Unstable Dark Matter: Spectral and Anisotropy Signatures in Astrophysical Backgrounds
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Le Zhang, UW-Madison/Hamburg University/DESY
Abstract: The nature of dark matter is one of the key outstanding problems in both particle and astrophysics. If dark matter decays or annihilates into electrons and positrons, it can affect diffuse radiation backgrounds observed in astrophysics. We present a new, more general analysis of constraints on dark matter models. For any decaying dark matter model, constraints on mass and lifetime can be obtained by folding the specific dark matter decay spectrum with a response function. These response functions are derived from full-sky radio surveys and Fermi-LAT gamma-ray observations and apply them to place constraints on some specific dark matter decay models. We also discuss the influence of astrophysical uncertainties on the response function, such as the uncertainties from propagation models and from the spatial distribution of the dark matter. <br>
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Moreover, an anisotropy analysis of full-sky emission gamma-ray and radio maps is performed to identify possible signatures of annihilating dark matter. We calculate angular power spectra of the cosmological background of synchrotron emission from dark matter annihilations into electron-positron pairs. We compare the power spectra with the anisotropy of astrophysical and cosmological radio backgrounds, from normal galaxies, radio-galaxies, galaxy cluster accretion shocks, the cosmic microwave background and Galactic foregrounds. In addition, we develop a numerical tool to compute gamma-ray emission from such electrons and positrons diffusing in the smooth host halo and in substructure halos with masses down to earth mass. We show that, unlike the total gamma-ray angular power spectrum observed by Fermi-LAT, the angular power spectrum from the inverse Compton scattering is exponentially suppressed below an angular scale determined by the diffusion length of electrons and positrons.
Host: Peter Timbie
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