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This Week at Physics

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Events During the Week of May 5th through May 12th, 2013

Monday, May 6th, 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, May 7th, 2013

Chaos & Complex Systems Seminar
Lessons learned from 19 years of chaos and complexity
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Clint Sprott, UW Department of Physics
Abstract: As we conclude the nineteenth year of the Chaos and Complex Systems Seminar, I would like to discuss some of the lessons I have learned from listening to over 500 talks, from my own research, and from the many books and articles I have read on the subject. This will be a rather personal and subjective talk and thus probably controversial. In particular, I will argue that the feedback, nonlinearities, and self-organization that characterize all real dynamical systems are more likely to ameliorate the dire consequences that others have predicted than to exacerbate them as so many fear. This is not a prediction that our problems will vanish or an argument for ignoring them. On the contrary, our choices and actions are the means by which society will reorganize to become even better in the decades to follow, albeit surely not a Utopia.
Host: Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
Magnetic Confinement in Plasma Physics
Time: 1:20 pm
Place: 2223 Chamberlin
Speaker: Karsten McCollam, 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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Wednesday, May 8th, 2013

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, May 9th, 2013

R. G. Herb Condensed Matter Seminar
Magnetization dynamics and semi-metal-insulator transition on the surface of a topological insulator
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Ilya Eremin, Ruhr University Bochum
Abstract: Due to its unique properties, topological insulators (TI) are likely to play a major role as a component material in different types of heterostructures. For instance, with a view towards spintronics applications, heterostructures involving ferromagnetic (FM) materials and topological insulators are extensively studied both theoretically and experimentally at present.

A thin film of ferromagnetically ordered material proximate to the surface of a three-dimensional topological insulator explicitly breaks the time-reversal symmetry of the surface states. For an out-of-plane ferromagnetic order parameter on the surface, parity is also broken, since the Dirac fermions become massive. This leads in turn to the generation of a Chern-Simons term by quantum fluctuations. On the other hand, for an in-plane magnetization the surface states remain metallic.

We consider a theory for a two-dimensional interacting conduction electron system with strong spin-orbit coupling on the interface between a topological insulator and the magnetic (ferromagnetic or antiferromagnetic) layer. For the ferromagnetic case we derive the Landau-Lifshitz equation, which features a contribution proportional to a fluctuation-induced electric field obtained by computing the topological (Chern-Simons) contribution from the vacuum polarization. We also show that fermionic quantum fluctuations reduce the critical temperature T*c at the interface relative to the critical temperature Tc of the bulk, so that in the interval T*c ≤T<Tc it is possible to have a coexistence of gapless Dirac fermions at the interface with a ferromagnetically ordered layer. For the case of an antiferromagnetic layer on a topological insulator substrate, we show that a second-order quantum phase transition occurs at the interface, and compute the corresponding critical exponents. In particular, we show that the electrons at the interface acquire an anomalous dimension at criticality. The critical behavior of the Néel order parameter is anisotropic and features large anomalous dimensions for both the longitudinal and transversal fluctuations.

In addition we study the possibility of spontaneous breaking of parity due to a dynamical gap generation on the surface. We find that in the absence of interaction between the fermions there is no spontaneous gap generation. In the presence of a local, Hubbard-like, interaction of strength g, a gap and a Chern-Simons term are generated for g larger than some critical value provided the number of Dirac fermions, N is odd. For an even number of Dirac fermions the masses are generated in pairs having opposite signs, and no Chern-Simons term is generated. Our result offers a possible explanation to recent experiments showing a gap opening even when the topological insulator is proximate to a planar ferromagnet.

[1] F.S. Noguiera and Ilya Eremin, Phys. Rev. Lett. 109, 237203 (2012)
[2] F.S. Noguiera and Ilya Eremin, arXiv:1304.2933 (unpublished).
Host: Chubukov
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WHITFORD LECTURE
The Virgo Cluster of Galaxies
Time: 3:45 pm
Place: Van Vleck B102 - Coffee served at 3:30 outside 4421 Sterling Hall
Speaker: Laura Farrarese, Herzberg Institute of Astrophysics, Univ of British Columbia
Abstract: At a distance of 16.5 Mpc and with a gravitating mass of 4.2A--10^14 solar masses, the Virgo Cluster is the dominant mass concentration in the local universe, the centre of the Local Supercluster, and the largest concentration of galaxies within ~35 Mpc. With thousands of member galaxies lying at a nearly common distance and spanning virtually all known morphological types, it has historically played a key role in studies of how galaxies form and evolve in dense environments. It is, without question, the most thoroughly studied cluster of galaxies in the universe, and remains a preferred target for a systematic survey of baryonic substructures in the low-redshift universe.

In this talk, I will describe an ambitious optical imaging survey of the Virgo cluster, the Next Generation Virgo Cluster Survey (NGVS), that is being carried out using the MegaPrime instrument at the Canada France Hawaii Telescope (CFHT). The NGVS is designed to address a wide range of fundamental astrophysical questions, including: the faint-end shape of the luminosity function, the characterization of galaxy scaling relations over a factor 10^7 in mass, the cluster/intracluster medium/galaxy connection, and the fossil record of star formation and chemical enrichment in dense environments. I will present a brief overview of the NGVS and discuss preliminary results.

Host: WOWSA.
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Friday, May 10th, 2013

Senior Sendoff Pizza Party
Time: 11:30 am
Place: Courtyard Between Chamberlin and Sterling
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Theory Seminar (High Energy/Cosmology)
Very Light Stops at the LHC
Time: 2:15 pm
Place: 5280 Chamberlin Hall
Speaker: David Morrissey, TRIUMF
Abstract: Supersymmetry can stabilize the weak scale against quantum corrections, produce a viable dark matter candidate, and provide an explanation for the excess of matter over antimatter. If supersymmetry is realized in Nature, for every known particle there exists a superpartner particle differing in spin by half a unit. Stops, the spin-zero superpartners of the top quark, play a key role in determining the phenomenological signals of supersymmetry. In this talk I will discuss the many reasons why at least one of the stops might have a mass only slightly above the weak scale, and I will describe some of the ways that such a light stop can be searched for and discovered at the LHC.
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Physics Department Colloquium
Gas-filled detectors: the Darwinian Champions of Particle Physics
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: David Nygren, Lawrence Berkeley National Laboratory
Abstract: The first detection of single ionizing events occurred more than 100 years ago when Ernest Rutherford and Hans Geiger succeeded in recording alpha particles from radon decay using a gas-filled detector and an electrometer. Remarkably diverse and useful innovations followed and continue to emerge even today, establishing gas-filled detectors as the exemplary evolutionary survivors in nuclear and particle physics technique. Although this ample record has many interesting chapters, I will focus on my favorite topics within this humble corner of the quest to understand our universe. The evolution of these devices is interesting not only for their substantial contribution to experiment, but also for what was, surprisingly, overlooked as technology evolved.
Host: Halzen
Poster: http://www.physics.wisc.edu/twap/posters/2013/2743.pdf
Video: http://www.physics.wisc.edu/vod/2013/05/10.html
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2013/2013-05-06.pdf

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