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This Week at Physics

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Events During the Week of April 10th through April 17th, 2011

Monday, April 11th, 2011

Plasma Physics (Physics/ECE/NE 922) Seminar
"Quasi-axisymmetric Tokamak: Breaking Tokamak Symmetry -- The Good, the Bad, and the Beautiful"
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Tony Taylor, General Atomics
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Tuesday, April 12th, 2011

R. G. Herb Condensed Matter Seminar
Interacting fermions on the honeycomb and its bilayer
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Oskar Vafek, Florida State University
Abstract: Electron-electron interaction effects on the graphene honeycomb lattice, and its AB stacked bilayer, will be compared. While there are no low temperature weak coupling instabilities of interacting massless Dirac fermions in 2D, such instabilities are unavoidable for two parabolically touching bands. We use weak-coupling renormalization group as well as strong-coupling expansion to determine the dominant ordering tendency for spinless and spin 1/2 fermions on the bilayer for models with different microscopic interactions. We find that for spinless fermions on the honeycomb bilayer the broken symmetry state is typically a gapped insulator with either broken inversion or broken time-reversal symmetry, with a quantized anomalous Hall effect (i.e., either a layer polarized state or an anomalous quantum Hall state). Additionally, a tight-binding model with nearest-neighbor hopping and nearest-neighbor repulsion is studied in weak and strong couplings and in each regime a gapped phase with inversion symmetry breaking is found. In the strong-coupling limit, the ground-state wave function can be constructed for vanishing in-plane hopping but finite interplane hopping, which explicitly displays the broken inversion symmetry and a finite difference between the number of particles on the two layers. For spin-1/2 fermions the resulting instabilities are studied as a function of the range of the electron-electron repulsion. For longer range interactions (several tens of lattice spacings) the dominant ordering tendency is towards an electronic nematic, while for short range repulsion (of order a lattice spacing as in a repulsive Hubbard model) the leading instability is found towards a Neel antiferromagnet.

[1] Oskar Vafek and Kun Yang, PRB 81, 041401 (2010). (Physics 3, 1 (2010))
[2] Oskar Vafek, PRB 82, 205106 (2010)
Host: Andrey Chubukov
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Chaos & Complex Systems Seminar
The Search for the Monsters at the Centers of Galaxies
Time: 12:05 pm
Place: 4274 Chamberlin
Speaker: Andy Sheinis, UW Department of Astronomy
Abstract: The past decade has given rise to conclusive evidence that all galaxies harbor a super-massive black hole in their cores. A black hole is a massive body whose surface gravity is so great that light cannot escape its gravitational pull. A growing understanding of the connection between galaxies and their central black holes has emerged that relates some of the properties of the black hole, whose gravity influences only the central 1/10,000 of the galaxy, to the global properties of the entire galaxy. Furthermore the latest galaxy formation and evolution theories require the input of energy from the black hole into the galaxy to achieve the size, shape and number density of the galaxies we observe today. These facts suggest that the growth mechanisms of the black hole and galaxy must be connected. However, details of the physical processes behind this connection are not yet understood. I will present an overview of the status of the field and then discuss my research to understand the nature of some of the most massive of these objects that are in the process of consuming massive amounts of matter from their host galaxies. These objects are Quasi-Stellar Objects or QSO's, which shine brightly in the sky due to the excess gas that escapes their feeding process.
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Wednesday, April 13th, 2011

Public Lecture at Wisconsin Institute for Discovery
The Birth of the Zooniverse: How Citizen Scientists are Taking On Research from Galaxies to Climate Change
Time: 5:30 pm
Place: Wisconsin Institute for Discovery Town Center
Speaker: Lucy Fortson, Associate Professor UMN Physics
Abstract: Imagine that you are an astronomer and you have a goldmine of data - one million galaxies digitally imaged by the Sloan Digital Sky Survey, the first-ever digital survey of the heavens. In amongst these galaxies are beautiful grand design spirals with blue knots of star formation regions beaded into the graceful spiral arms. There are also giant elliptical galaxies that are mostly red as their star formation died out ages ago leaving the glowing embers of a population of older stars. As an astronomer, you know that there are still many unanswered questions about why this dichotomy between blue spirals and red ellipticals exists and how galaxies evolved to be this way from the Big Bang. One question you might ask yourself is whether, hidden amongst these million galaxies, are images of blue ellipticals or red spirals. That is, are there elliptical galaxies that still have enough star formation occurring within their vast reaches to appear significantly blue to our digital instruments; or are there spiral galaxies where for some unknown reason, all the stars are from an older generation of stars and thus appear red? Now, what if you were told that no computer code could be written to find these blue ellipticals or red spirals because the shapes were too complex for a computer to decipher? That the only way to solve this problem was to look at each of the million galaxies by eye and classify them one-by-one? This is the problem that was presented to a graduate student at Oxford University in 2006 and his solution to the problem resulted in the birth of the now-famous Galaxy Zoo project. Galaxy Zoo asked the general public to volunteer their time and their visual cortex to look at the images of these million galaxies through an online interface. No astronomy training was required or wanted - just the pattern matching skills at which humans are so good and computers so terrible. Galaxy Zoo was a tremendous success with more than 100,000 volunteers providing over 60 million classifications leading to over 20 papers published in scientific journals. Based on this success, the Galaxy Zoo team developed several more projects where this `citizen science' method of data processing could be applied. And thus the `Zooniverse' was born. In this presentation, Dr. Fortson will overview the Galaxy Zoo project, describing the method of citizen science data processing and how this method eventually marries humans with computers to gain the most knowledge out of the huge flood of digital data that continues to pour in across all disciplines. Along the way, she will talk about some of the discoveries made by the general public and demonstrate several of the current projects available to the more than 400,000 volunteers in the Zooniverse.
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Thursday, April 14th, 2011

R. G. Herb Condensed Matter Seminar
Interface engineering in epitaxial oxide heterostructures
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Alex Demkov, University of Texas at Austin
Abstract: The astounding progress of recent years in the area of oxide deposition has made possible the creation of oxide heterostructures with atomically abrupt interfaces. The ability to control the length scale, strain, and orbital order in these materials structures offers a uniquely rich toolbox for condensed matter physicists. Because the oxide layers are very thin, the physics is often controlled by the interface. The electronic properties of oxide interfaces are governed by a subtle interplay of many competing mechanisms such as polar catastrophe, Jahn-Teller coupling, electron correlation, defects, and phase stability. It is not clear which, if any, of these systems will find applications in future high-tech devices. However, they undoubtedly hold tremendous promise, particularly when integrated with conventional semiconductors such as Si.

In this talk I will review our recent results in theoretical modeling and experimental realization of several epitaxial oxide heterostructures. I will set the stage with a brief discussion of the n-type conductive SrTiO3/LaA1O3 interface [1]. The origin of charge in this nominally insulating system is still under intense debate, and the difficulties are, in my opinion, generic to the entire field. Our theoretical results are consistent with the originally proposed polar catastrophe model, should the field be indeed stabilized in polar LaA1O3. However, our results indicate that this may be difficult to achieve. I then will discuss extrinsic magnetoelectric coupling at the interface of a perovskite ferroelectric and conventional ferromagnet [2,3]. In contrast with the previously proposed models, the structure we consider offers the robust, linear coupling. If time permits, I will describe our efforts to achieve the monolithic integration of ferromagnetic oxide LaCoO3(LCO) and silicon for possible applications in spintronics [4]. The integration is achieved via the single crystal SrTiO3 (STO) buffer epitaxially grown on Si. The intermediate spin state is stabilized by epitaxial strain at the STO/LCO interface.

1. J. K. Lee and A. A. Demkov, Phys. Rev. B 78, 193104 (2008).
2. T. Cai, Q. Niu, J. K. Lee, Na Sai, and A. A. Demkov, Phys. Rev. B 80, 140415(R) (2009).
3. J. K. Lee, Na Sai, T. Cai, Q. Niu and A. A. Demkov, Phys. Rev. B 81, 144425 (2010).
4. A. Posadas, M. Berg, H. Seo, D. J. Smith, H. Celio, A. P. Kirk, D. Zhernokletov, R. M. Wallace, A. de Lozanne, and A. A. Demkov, Appl. Phys. Lett. 98, 055104 (2011).
Host: Susan Coppersmith & Irena Knezevic
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Astronomy Colloquium
The new world of Gamma Ray Astronomy
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Lucy Fortson, University of Minnesota
Abstract: With the third generation ground-based gamma-ray telescopes delivering over a hundred new TeV emitting objects and with the new Fermi satellite providing greatly improved sensitivity in the GeV energy regime, gamma ray astronomy is entering a golden age. I will first review the basics of ground-based gamma-ray astronomy and the Air Cherenkov Telescope method of detection. I will then describe VERITAS - an array of four gamma ray telescopes located at Mt. Hopkins, Arizona - and some of the recent results from the first few years of the VERITAS observing program, paying attention to the observations of several new TeV emitting active galactic nuclei. I will close with a look at the future of ground-based gamma ray astronomy with the CTA - the proposed array of 36-100 Cherenkov Telescopes.
Host: Prof Andrew Sheinis
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Friday, April 15th, 2011

Phenomenology Seminar
Theory/Phenomenology Seminar
CP Violation in Bs Mixing in the MSSM and Beyond
Time: 2:30 pm
Place: 5280 Chamberlin Hall
Speaker: Wolfgang Altmannshofer, Fermilab
Abstract: Recently there has been considerable progress in the experimental determination of the Bs mixing phase. Data from the Tevatron seem to point towards large CP violation in Bs mixing at the level of 3 sigma above the tiny SM prediction. I will briefly review the current experimental status and then discuss if and how large CP violation in Bs mixing can be accommodated for in various supersymmetric models, including the MSSM with Minimal Flavor Violation and supersymmetric flavor models that show representative flavor structures in the soft SUSY breaking terms.
Host: Maike Trenkel
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Physics Department Colloquium
Searching for Inflation from the South Pole with CMB Polarimetry
Time: 4:00 pm
Place: 2241 Chamberlin Hall (coffee at 3:30 pm)
Speaker: John Kovac, Harvard University
Abstract: Inflation predicts a cosmic gravitational-wave background (CGB), the amplitude of which measures the inflationary energy scale. The CGB in turn produces a faint but unique signature in the 'B-mode' polarization of the cosmic microwave background (CMB). BICEP1, the first experiment specifically designed to search for this signature, began observing from the South Pole in early 2006. It has produced the highest sensitivity measurements yet made of CMB polarization at the ~2 degree angular scales where the inflationary signal is expected to peak. Follow-on experiments BICEP2 and the Keck Array have now improved sensitivity dramatically are currently testing models of inflation at the GUT scale. At smaller angular scales, measurements of B-modes from gravitational lensing promise a sensitive probe of the Dark Energy equation of state, the sum of neutrino masses, and improved constraints on inflation. This decade will see intensified efforts to probe a rich array of fundamental physics through CMB polarization.
Host: Karle
Poster: http://www.physics.wisc.edu/twap/posters/2011/2022.pdf
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2011/2011-04-11.pdf

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