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This Week at Physics

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Events on 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>
<br>
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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