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This Week at Physics

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Events on Thursday, March 10th, 2011

R. G. Herb Condensed Matter Seminar
Topological Insulators
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Liang Fu, Harvard University
Abstract: Traditionally phases of matter are classified by spontaneous symmetry breaking. The discovery of quantum Hall effect in 1980s led to the concept of topological phases characterized by topological quantum numbers. In the past few years, new topological phases have been theoretically predicted and experimentally observed in band insulators at zero magnetic field. These topological insulators are characterized by topological invariants in the band structure. They have protected gapless surface states with Dirac dispersion. I will describe theory of topological insulators, prediction and observation of these new phases in real materials, and recent experiments on topological surface states. I will end by describing recently observed superconductivity in doped topological insulators and its implication for unconventional pairing in spin-orbital coupled materials.
Host: Andrey Chubukov & Maxim Vavilov
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Astronomy Colloquium
"The Formation of Molecular Clouds and Massive Stars"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Mordecai-Mark Mac Low, American Museum of Natural History
Abstract: In this talk I consider two questions. First, I investigate the formation of molecular clouds from diffuse interstellar gas. It has been argued that the midplane pressure controls the fraction of molecular hydrogen present, and thus the star formation rate. Alternatively, I and others have suggested that the gravitational instability of the disk controls both. I present numerical results demonstrating that the observed correlations between midplane pressure, molecular hydrogen fraction, and star formation rate can be explained within the gravitational instability picture. Second, I discuss how ionization affects the formation of massive stars. Although most distinctive observables of massive stars can be traced back to their ionizing radiation, it does not appear to have a strong effect on their actual formation. Rather, I present simulations suggesting that stars only ionize large volumes after their accretion has already been throttled by gravitational fragmentation in the accretion flow. At the same time these models can explain many aspects of the observations of ultracompact H II regions.&lt;br&gt;<br>
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Host: Astronomy Department
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