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Events on 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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