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Events on Thursday, December 15th, 2011

Last Day of Class
R. G. Herb Condensed Matter Seminar
Orbital and magnetic ordering in ZnV2O4
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Yasuyuki Kato, Los Alamos National Laboratory
Abstract: Spinel group comprise corner sharing tetrahedra and constitute paradigmatic examples of geometrically frustrated lattices. The spinel compound, ZnV2O4, is a Mott insulator with a rather small charge gap that undergoes a structural cubic to tetragonal transition at T=50(K). A magnetic transition at a lower temperature of T=40(K) has also been reported for this compound. The magnetic ordering corresponds to up-up-down-down spin configurations for chains oriented along the yz and zx directions. The origin of this magnetic ordering and the lack of orbital ordering in this material has been an open problem for several decades. The main obstacle was the lack of controlled and unbiased approaches for solving the correlated and frustrated model. I will introduce a novel quantum Monte Carlo method that can simulate the three-band Hubbard model relevant for these materials without generating any serious problem. We will see that this unbiased approach not only explains the observed magnetic ordering but also the lack of orbital ordering.
Host: Perkins
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Astronomy Colloquium
AGB Dust Production in the Local Group
Time: 3:30 pm - 5:00 pm
Place: 4421 Sterling Hall
Speaker: Martha Boyer, Space Telescope Science Institute
Abstract: There are two known ways to create new dust to input into the interstellar medium: in supernova explosions and in the circumstellar envelopes of evolved stars. Massive evolved stars such as red supergiants produce a lot of dust, but Asymptotic Giant Branch (AGB) stars are thought to be the dominant stellar creators of dust at both low and high redshift (z), possibly even in the host galaxy of one of the most distant quasars (z=6.4; Valiante et al. 2009). Using observations from the Spitzer Space Telescope, we can for the first time study entire populations of AGB stars and quantify the total AGB dust production in Local Group Galaxies. Here, I discuss recent work on dust production in nearby globular clusters and dwarf galaxies, with particular focus on the Magellanic Clouds.
Host: Barbara Whitney
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