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This Week at Physics

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Events During the Week of January 27th through February 3rd, 2013

Monday, January 28th, 2013

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:30 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
    http://cmb.physics.wisc.edu/journal/index.html
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Le Zhang (lzhang263@wisc.edu)
Host: Peter Timbie
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Tuesday, January 29th, 2013

Chaos & Complex Systems Seminar
What do smiles mean and how do we know?
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Paula M. Niedenthal, UW Department of Psychology
Abstract: Theories of embodied emotion suggest new ways to model the recognition of facial expression. Behavioral and neuroimaging studies indicate that the recognition of facial expressions of emotion, and in particular the elusive smile, involves the (re)production of the expression as well as of the corresponding emotion, or parts of it, in the self. In the present talk, I introduce a new model, The Simulation of Smiles Model (SIMS, Niedenthal et al., BBS, 2010). The SIMS relies on a social-functional typology of smiles. Accordingly, I first present research that seeks to validate the typology. The SIMS also outlines specific roles for facial mimicry and eye contact in representing smile meaning. Recent empirical evidence in favor of these roles is presented. Finally, the SIMS leaves room for the use of perceptual and conceptual processes in interpreting facial expression. I present research supportive of the hypothesis that the interpretation of smile meaning relies on prior beliefs and stereotypes when facial mimicry does not occur. Results of a recent study on smiling behavior from 9 different countries provides the basis for predictions about moderation by culture of the basic processes outlined in SIMS.
Host: Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
TeV Gamma-ray Astronomy with HAWC
Time: 1:20 pm
Place: 2223 Chamberlin
Speaker: Stefan Westerhoff, University of Wisconsin Department of Physics
Abstract: A series of weekly presentations and discussions of current research topics in physics by the scientists involved in those studies designed to expose students to the topics and excitement of the research frontier.
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Theory Seminar (High Energy/Cosmology)
Measuring top-quark polarization in top-pair + missing-energy events
Time: 4:00 pm
Place: 5280 Chamberlin Hall
Speaker: Hao Zhang, Argonne National Laboratory/Illinois Institute of Technology
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Wednesday, January 30th, 2013

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, January 31st, 2013

R. G. Herb Condensed Matter Seminar
Electronic Transport in Bi2Se3 Thin Films in the Topological Insulator Regime
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Dohun Kim, University of Maryland
Abstract: The 3D topological insulators (TIs) have an insulating bulk but metallic surface states stemming from band inversion due to strong spin-orbit interaction, whose existence is guaranteed by the topology of the band structure of the insulator. Like graphene, the STI surface state generically has a Dirac electronic spectrum with massless electrons and a vanishing bandgap at a Dirac point. In this talk, I will discuss experiments on the TI material Bi2Se3, which has a single topological Dirac surface state. Field effect transistors consisting of thin (5-17 nm) Bi2Se3 are fabricated by mechanical exfoliation of single crystals, and electrochemical and/or chemical gating methods are used to move the Fermi energy into the bulk bandgap, revealing the ambipolar gapless nature of transport in the Bi2Se3 surface states [1]. The minimum conductivity of the topological surface state is understood within the self-consistent theory of Dirac electrons in the presence of charged impurities. The intrinsic finite-temperature resistivity of the topological surface state due to electron-acoustic phonon scattering is measured to be 60 times larger than that of graphene largely due to the smaller Fermi and sound velocities in Bi2Se3 [2], which will have implications for topological electronic devices operating at room temperature. I will also discuss about our recent observation of 2D weak anti-localization (WAL) behavior in the low field magneto transport, which stems from topological surface states. By investigating gate-tuned WAL behaviors, I will show that WAL in TI regime is extraordinarily sensitive to sub-meV coupling between top and bottom topological surfaces, and interplay of phase coherence time and inter surface tunneling time results interesting crossovers from coupled single channel to decoupled multichannel coherent transports.

[1] D. Kim et al., Nature Phys. 8, 460 (2012).
[2] D. Kim et al., Phys. Rev. Lett. 109, 166801 (2012).
Host: Eriksson
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Astronomy Colloquium
Unambigious tests of general relativity with Xray Spectroscopy of AGN
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Guido Risaliti, Harvard Smithsonian Cfa/Observatory of Florence
Abstract: <br>
<br>
Active Galactic Nuclei (AGN) are optimal laboratories for tests of general relativity (GR) in the &quot;strong field&quot; regime. In particular their X-ray emission comes from a few gravitational radii from the horizon event of the supermassive black hole, and is strongly affected by GR effects. However, testing GR has been difficult so far because of a strong degeneracy in the X-ray spectra of AGN between the (relativistically distorted) intrinsic emission and the complex absorption/reflection components due to the circumnuclear medium. I will review the main diagnostic methods developed in the past few years to investigate this issue, focusing on time-resolved spectral analysis of variable sources, and I will discuss the impact in this field of the recently launched hard X-ray telescope NuSTAR. I will show how the availability of high quality broad-band X-ray spectra, and a time-resolved spectral analysis can remove the main systematic uncertainties in GR tests, provide precise measurements of black hole spins, and reveal the structure of the circumnuclear X-ray absorber.
Host: Prof Elena D'onghia
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Friday, February 1st, 2013

Physics Department Colloquium
Femtosecond hard X-ray lasers for atomic structure and biodynamics
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: John Spence, Arizona State University
Abstract: Since the Linac Coherent Light Source (LCLS) started operation in late 2009 at SLAC we have collected femtosecond pulsed coherent X-ray scattering from many molecular systems. It has been found that sufficiently brief X-ray pulses terminate before radiation damage commences, opening up many opportunities for new experiments in time- resolved imaging with atomic spatial resolution at room temperature, in condensed matter physics, materials science and biology.

I will review some of these, including pump-probe experiments on the large molecular complexes involved in photosynthesis, and on a drug target molecule for sleeping sickness. A new approach to disentangling orientational disorder will also be demonstrated, aimed at reconstructing the image of one molecule, using the scattering from many in random orientations in solution, without modeling, based on angular correlation functions. Prospects for the formation of "molecular movies" which track chemical reactions will be outlined. I'll also describe the new approaches to the phase problem which these experiments suggest. A review of all this work can be found in Rev Mod Phys. 75, 102601 (2012).
Host: Gilbert
Poster: http://www.physics.wisc.edu/twap/posters/2013/2708.pdf
Video: http://www.physics.wisc.edu/vod/2013/02/01.html
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2013/2013-01-28.pdf

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