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This Week at Physics

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Events During the Week of April 22nd through April 29th, 2012

Monday, April 23rd, 2012

ECE 600 Seminar: Silicon Nanomembrane based Photonic Crystal Nanostructures for On-chip Open Sensor Systems
ECE 600 Seminar: Silicon Nanomembrane based Photonic Crystal Nanostructures for On-chip Open Sensor Systems
Time: 3:30 pm
Place: Engineering Hall, Room 1800
Speaker: Prof. Ray Chen, University of Texas-Austin
Abstract: We experimentally demonstrate a myriad of devices on the silicon nanmomembrane based photonic crystal platforms for chip-integrated optical absorption spectroscopy and chip-integrated biomolecular microarray assays. Infrared optical absorption spectroscopy and biomolecular assays based on conjugate-specific binding principles represent two dominant sensing mechanisms for a wide spectrum of applications in environmental pollution sensing in air and water, chem-bio agents and explosives detection for national security, microbial contamination sensing in food and beverages to name a few. The easy scalability of photonic crystal devices to any wavelength ensures that the sensing principles hold across a wide electromagnetic spectrum. Silicon, the work horse of the electronics industry, is an ideal platform for the above optical sensing applications.
Host: Prof. Luke Mawst
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Condensed Matter Theory Group Seminar
Two-electron dephasing in single Si and GaAs quantum dots
Time: 4:30 pm
Place: 5310 Chamberlin
Speaker: John Gamble, UW-Madison
Abstract: We study the dephasing of two-electron states in a single quantum dot in both GaAs and Si. We investigate dephasing induced by electron-phonon coupling and by charge noise analytically for pure orbital excitations in GaAs and Si, as well as for pure valley excitations in Si. In GaAs, polar optical phonons give rise to the most important contribution, leading to GHz dephasing rates. For Si, intervalley optical phonons lead to typical dephasing rates of ~100 kHz for orbital excitations and ~1 MHz for valley excitations. For harmonic, disorder-free quantum dots, charge noise is highly suppressed for both orbital and valley excitations, since neither has an appreciable dipole moment to couple to electric field variations from charge fluctuators. However, both anharmonicity and disorder break the symmetry of the system, which can lead to increased dipole moments and therefore faster dephasing rates.
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Tuesday, April 24th, 2012

Chaos & Complex Systems Seminar
Adaptive information
Time: 12:05 pm
Place: 4274 Chamberlin
Speaker: Rob Nowak, UW Department of Electrical and Computer Engineering
Abstract: This talk will discuss the notions of adaptive and non-adaptive information in the context of statistical learning and inference. Suppose that we have a collection of models (e.g., signals, systems, representations, etc.) denoted by X and a collection of measurement actions (e.g., samples, probes, queries, experiments, etc.) denoted by Y. A particular model x in X best describes the problem at hand and is measured as follows. Each measurement action, y in Y, generates an observation y(x) that is a function of the unknown model. This function may be deterministic or stochastic. The goal is to identify x from a set of measurements y_1(x),...,y_n(x), where y_i in Y, i=1,...,n. If the measurement actions y_1,...,y_n are chosen deterministically or randomly without knowledge of x, then the measurement process is non-adaptive. However, If y_i is selected in a way that depends on the previous measurements y_1(x),...,y_{i-1}(x), then the process is adaptive. Adaptive information is clearly more flexible, since the process can always disregard previously collected data. The advantage of adaptive information is that it can sequentially focus measurements or sensing actions to distinguish the elements of X that are most consistent with previously collected data, and this can lead to significantly more reliable decisions. The idea of adaptive information gathering is commonplace (e.g., humans and animals excel at this), but outside of simple parametric settings little is known about the fundamental limits and capabilities of such systems. Some preliminary results addressing this situation will be described.
Host: Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
The Origin of Matter
Time: 1:20 pm
Place: 2223 Chamberlin
Speaker: Ramsey-Musolf, University of Wisconsin Department of Physics
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High Energy Seminar
Experimental Muon Physics: an Overview
Time: 3:00 pm
Place: 5310 Chamberlin Hall
Speaker: David Webber, University of Wisconsin Madison
Abstract: According to the Standard Model of particle physics, muons behave like
heavy electrons which decay only via the weak force. Their properties
make them especially well-suited to precision measurements of
fundamental parameters of the Standard Model, as well as searches for
physics beyond the Standard Model. Muons have the longest lifetime of
any fundamental particle which still decays, allowing them to form
chemical bound states and probe the structure of the proton.
Additionally, muon decay maximally violates parity, correlating the
decay electron with the muon's spin and enabling measurements of g-2.
In this talk, several recent experimental results will be reviewed,
and some upcoming experiments will be presented.
Host: Karsten Heeger
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Wednesday, April 25th, 2012

No events scheduled

Thursday, April 26th, 2012

R. G. Herb Condensed Matter Seminar
"Rare" Fluctuations and the Anderson Model of Localization
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Ravin Bhatt, Princeton University
Abstract: It has been over fifty years since Anderson's original paper reporting the discovery of the absence of diffusion in random lattices, and over three decades since the metal-insulator transition was understood in terms of the scaling theory of localization. Despite this long time interval, and thousands of publications on the subject, the Anderson model of localization continues to offer new insights. In this talk we examine the Anderson model at large disorder (i.e. in the insulating phase). We find, surprisingly, new singularities in this regime, which had apparently escaped attention. These singularities appear to demarcate the boundary between "typical" and "rare fluctuation" states. (The latter are the counterpart of rare fluctuation effects found to be quite pervasive in quantum many-body models with large disorder). Besides exposing a new facet of a seemingly simple and extensively studied model, our work suggests that Anderson's model of localization offers an unparalleled opportunity to understand rare fluctuation effects at a level of detail that has not been possible in numerical approaches to many-body models.
Host: Susan Coppersmith
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Dark Matter from Weak Polyplets
Time: 2:30 pm
Place: 4274 Chamberlin
Speaker: Jennifer Kile, Northwestern University
Abstract: We investigate the possibility of new fermion multiplets charged under the Standard Model gauge group, with the aim of obtaining a possible dark matter candidate. These new fermions are charged under SU(2)xU(1); their quantum numbers are determined by requiring anomaly cancellation and insisting that all new particles become massive via Yukawa couplings with the SM Higgs boson. Constraints from colliders, electroweak precision measurements, and DM direct detection are considered; we find that this model can accommodate a viable DM candidate.
Host: Michael Ramsey-Musolf
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Friday, April 27th, 2012

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:00 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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Physics Department Colloquium
Topological insulators and topological superconductors
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Shoucheng Zhang, Stanford University
Abstract: Recently, a new class of topological states has been theoretically predicted and experimentally observed. The topological insulators have an insulating gap in the bulk, but have topologically protected edge or surface states due to the time reversal symmetry. Similarly, topological superconductors or superfluids have novel edge or surface states consisting of Majorana fermions. In this talk, I shall review the recent theoretical and experimental progress in the field, and focus on a number of outstanding issues, including the quantized anomalous Hall effect, quantized magneto-electric effect, the topological Mott insulators and the search for topological superconductors and Majorana fermions.
Host: Coppersmith
Poster: http://www.physics.wisc.edu/twap/posters/2012/2295.pdf
Video: http://www.physics.wisc.edu/vod/2012/04/27.html
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2012/2012-04-23.pdf

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