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This Week at Physics

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Events During the Week of April 24th through May 1st, 2011

Monday, April 25th, 2011

Plasma Physics (Physics/ECE/NE 922) Seminar
"Reducing turbulent transport in toroidal configurations through shaping"
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Harry Mynick, Princeton Plasma Physics Laboratory
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Tuesday, April 26th, 2011

Chaos & Complex Systems Seminar
"Paradise Lost? Teaching About Climate Change in the Great Lakes Region"
Time: 12:00 pm
Place: 4274 Chamberlin Hall
Speaker: Dolly Ledin, UW Center for Biology Education
Abstract: This unique outreach and education project brings together the compelling evidence of science, the interpretive talents of professional artists and the skills of educators to engage communities in learning about climate change in the Great Lakes region. Artists, scientists and educators collaborated to create a traveling multi-media exhibit, educational events and website for teachers.
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Wednesday, April 27th, 2011

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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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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Friday, April 29th, 2011

Physics Department Colloquium
Distinguished Award Winner
Atomic Clocks: Does Anybody Really Know What Time It Is?
Time: 4:00 pm
Place: 2241 Chamberlin Hall (coffee at 3:30 pm)
Speaker: Thomas R. O'Brian, National Institute of Standards and Technology (NIST) and JILA
Abstract: Time is the most accurately measured absolute quantity. The world's best atomic clocks at the National Institute of Standards and Technology (NIST) measure time with absolute uncertainties about 8 x 10-18, the equivalent of one second in 4 billion years. At this precision, relativistic time dilation is evident at jogging speeds or 10 cm changes in altitude. We will discuss how atomic timekeeping underpins a broad range of our technology infrastructure, enables innovative measurements for everything from brain activity to mineral exploration - and how related to atomic timekeeping has stimulated some of the most important advances in atomic and optical physics.
Host: Department of Physics
Poster: http://www.physics.wisc.edu/twap/posters/2011/1873.pdf
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2011/2011-04-25.pdf

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