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This Week at Physics

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Events During the Week of September 9th through September 16th, 2012

Monday, September 10th, 2012

Plasma Physics (Physics/ECE/NE 922) Seminar
Instabilities of a magnetized free shear layer in rotating liquid metal
Time: 12:00 pm
Place: 1310 Sterling Hall
Speaker: Austin Roach, Princeton Plasma Physics Laboratory
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Tuesday, September 11th, 2012

Chaos & Complex Systems Seminar
Computational complexity theory -- The world of P and NP
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Jin-Yi Cai, UW Department of Computer Science
Abstract: Computational Complexity Theory is the study of intrinsic difficulties of computational problems. The most prominent open problem is the conjecture that P is not equal to NP. In essense this conjecture states that it is intrinsically harder to find proofs than to verify them. It has a fundamental importance in many areas from computer science to mathematics, to our basic understanding of nature.<br>
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Valiant's new theory of holographic algorithms is one of the most beautiful ideas in algorithm design in recent memory. It gives a new look on the P versus NP problem. In this theory, information is represented by a superposition of linear vectors in a holographic mix. This mixture creates the possibility for exponential sized cancellations of fragments of local computations. The underlying computation is done by invoking the Fisher-Kasteleyn-Temperley method for counting perfect matchings for planar graphs (Dimer Problem). Holographic algorithms challenge our conception of what polynomial time computation can do, in view of the P vs. NP question.<br>
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In this talk we will survey the developments in holographic algorithms. No specialized background is assumed.
Host: Sprott
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Wednesday, September 12th, 2012

R. G. Herb Condensed Matter Seminar
How strain unintentionally causes quantum dots in silicon nano-devices
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Ted Thorbeck, National Institute of Standards and Technology
Abstract: Silicon quantum dots are useful both in making a quantum computer and in charge pumps for electrical standards. A common problem in the Si-SiO2 quantum dot community is that in addition to the electrostatically defined dots we get additional, unintentional dots. In this talk I will discuss how we have been able to determine the location of these unintentional dots within our devices with a precision of a few nanometers. I will then discuss how strain from the gates and the oxide causes unintentional dots in our devices, as well as for other groups working on Si-SiO2 quantum dots. This allows me to recommend methods to reduce the strain, thereby reducing the number of unintentional dots.
Host: McDermott
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Thursday, September 13th, 2012

R. G. Herb Condensed Matter Seminar
Competing Phases of 2D Electrons at ν = 5/2 and 7/3 in fractional quantum Hall systems
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Jing Xia, University of California - Irvine
Abstract: In 2 dimensional electron systems (2DES) in GaAs/AlGaAs quantum wells, the N=1 Landau level (LL) exhibits collective electronic phenomena characteristic of both fractional quantum Hall (FQHE) states seen in the lowest LL and anisotropic nematic states in the higher LLs. In particular, fractional quantum Hall effect (FQHE) at I1/2 = 5/2 (and 7/2) is thought to obey non-Abelian statistics and holds the promise of realizing topologically protected quantum computers. Intriguingly, a modest in-plane magnetic field B|| is sufficient to destroy the FQH states at I1/2 = 5/2 (and 7/2) and replace them with anisotropic compressible nematic phases, revealing the close competition between the two. We find that at larger B|| these anisotropic phases I1/2 = 5/2 can themselves be replaced by a new isotropic state, dubbed re-entrant isotropic compressible (RIC) phase [1]. We present strong evidence that this transition is a consequence of the mixing of Landau levels from different electric subbands in the confinement potential. In addition, we find that with B||, the normally isotropic I1/2 = 7/3 FQHE state can transform into an anisotropic phase with an accurately quantized Hall plateau but an anisotropic longitudinal resistivitiescite [2]. As temperature is lowered towards zero, Ixx diminishes while Iyy tends to diverge, reminiscent of the anisotropic nematic states, while surprisingly Ixy and Iyx remain quantized at 3h/7e2, indicating a completely new quantum phase.

References:
[1] J. Xia, V. Cvicek, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys Rev Lett 105, 176807 (2010).
[1] J. Xia, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Nature Physics (2011).
Host: Perkins
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Astroparticle physics with the ARGO-YBJ detector
Time: 2:30 pm
Place: 4274
Speaker: Dr. Roberto Iuppa, University of Rome "Tor Vergata", Roma, Italy
Abstract: The ARGO-YBJ experiment at YangBaJing in Tibet (4300 m a.s.l.) has been taking data with its full layout since November 2007. A few significant results obtained in gamma-ray astronomy and cosmic-ray physics are presented. Emphasis is placed on the analysis of gamma-ray emission from point-like sources, on the limit on the antiproton/proton flux ratio, on the measurement of the Mean Interplanetary Magnetic field, on the large-scale cosmic-ray anisotropy and on the protonaEuro"air cross-section. The performance of the detector is also discussed, and the perspectives of the experiment are outlined.
Host: Paolo Desiati
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Astronomy Colloquium
"Large-Scale Surveys of Star Formation in the Milky Way"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Peter Barnes, University of Florida
Abstract: The formation of massive stars and star clusters is still poorly understood, despite its importance in cosmology, ecology of the ISM, and stellar demographics. We still debate such basic questions as the main formation mechanism and the timescales. However, a wealth of data from several new surveys promises to transform our understanding of this process. I will present background and new results from two large-scale surveys of molecular gas and star formation content of the Milky Way's 4th quadrant. CHaMP is a multi-wavelength, sensitive, unbiased, and uniform study of all massive star formation sites at sub-parsec resolution within a 20x6 degree window in Vela, Carina, &amp;amp; Centaurus, including both the cold molecular gas and warmer areas heated by embedded young star clusters. The CHaMP clouds show a range of unexpected but key properties that shed new light on molecular cloud evolution and star cluster formation. ThrUMMS completely maps the remaining 60x1 portion of the 4th quadrant in 12CO, 13CO, C18O, and CN at arcminute- (ie, parsec-scale) resolution, and will be a key tool for obtaining distances to structures revealed by GLIMPSE, Hi-GAL, and many other surveys, as well as characterizing the physics of GMCs in more detail than possible previously. Both surveys feature freely downloadable image &amp;amp; data files for custom analysis and other applications.
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Prospective Undergraduate Majors in Physics
PUMP Meeting
Time: 4:00 pm
Place: 2241 Chamberlin Hall (Food & Soda will be served)
Abstract: Are you interested in majoring in Physics? Wondering what can you do with a Physics Degree? Come to the Prospective Undergraduate Majors in Physics (PUMP) Meeting to learn more.<br>
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Topics Include: Why Major in Physics; What Can You do with a Physics Degree; Major Requirements; Undergraduate Research Opportunities; and Tutoring &amp; Mentoring Opportunities.
Poster: http://www.physics.wisc.edu/twap/posters/2012/2766.pdf
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Graduate Introductory Seminar
Astrophysics I: Experimental Astro/Space Physics
Time: 5:45 pm
Place: 2223 Chamberlin Hall
Speaker: McCammon, Timbie, UW Madison
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Friday, September 14th, 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
Magnetic Reconnection in Plasmas: a Celestial Phenomenon in the Laboratory
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Jan Egedal, MIT Dept. of Physics & Plasma Science and Fusion Center
Abstract: Coronal mass ejections from the sun are the most explosive events that occur in our solar system. Closer to home, on earth the aurora borealis is a spectacular, naturally occurring, light show. Both of these large scale events are driven by magnetic reconnection in plasmas. The spontaneous rearrangement of magnetic field topology provides the enormous energy needed for these celestially magnificent and diverse phenomena.

Magnetic reconnection has been a fascinating topic of research in plasma physics for over sixty years. While we still do not fully understand the process of reconnection, significant progress has been made in the past decade through detailed analysis of laboratory experiments and computer simulations. The Versatile Toroidal Facility (VTF) at MIT is an experiment dedicated to the study of magnetic reconnection.

In this talk I will describe experimental observations from VTF which have led to a new theoretical paradigm for magnetic reconnection. Large scale computer simulations support the experimental and theoretical results detailing the release of magnetic energy during reconnection. We are now able to explain the large scale electron heating observed during reconnection by spacecraft in the earth's magnetotail. Our model may also provide insight to magnetic reconnection on the sun and the associated heating in solar flares and coronal mass ejections
Host: Forest
Poster: http://www.physics.wisc.edu/twap/posters/2012/2707.pdf
Video: http://www.physics.wisc.edu/vod/2012/09/14.html
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2012/2012-09-10.pdf

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