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This Week at Physics

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Events on Friday, February 4th, 2011

Astronomy Colloquium
Special Astronomy Colloquium
Direct Imaging of Exoplanets: Prospects for comparative Exoplanetology
Time: 3:15 pm
Place: 4421 Sterling Hall
Speaker: Beth Biller, MPIA Heidelberg
Abstract: Direct detection, and direct spectroscopy in particular,
have great potential for advancing our understanding of extrasolar
planets. In combinations with other methods of planet detection,
direct imaging and spectroscopy will allow us to eventually: 1) fully map out the architecture of typical planetary systems and 2) study the physical properties of exoplanets (colors, temperatures, etc.) in depth. I will discuss initial results from the ongoing 500 hour NICI Planet-Finding Campaign using the novel Near-IR Coronagraphic Imager (NICI) at the 8-m Gemini South telescope. NICI combines a number of techniques to attenuate starlight a suppress superspeckles for direct detection of exoplanets: 1) Lyot coronagraphic imaging, 2) dual channel imaging for Spectral Differential Imaging (SDI) and 3)operation in a fixed Cassegrain rotator mode for Angular Differential Imaging (ADI). The combination of these techniques allows unprecedented contrasts of dmag > 14 (median value) at 1" in H band, sufficient to image giant planets (<5 MJup) around stars in nearby young moving groups and super jupiters (<10 MJup) around stars in the immediate solar neighborhood. I will also discuss the discovery of a tight substellar companion to the young solar analog PZ Tel(Biller et al. 2010), a member of the &#946; Pic moving group observed as part of the Gemini Near-Infrared Coronagraphic Imager Planet-Finding Campaign.

PZ Tel B is one of the few young substellar companions
directly imaged at orbital separations similar to those of giant
planets in our own solar system.

Planetary mass companions to brown dwarfs are an important counterpart to planets around stars and provide key benchmark objects for evolutionary models of substellar objects. Therefore, I will also discuss results from a systematic Keck Laser Guide Star (LGS) adaptive optics search to directly image planetary mass companions to young brown dwarfs in the Upper Sco embedded cluster (Biller et al. 2011).
Host: Prof Amy Barger
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Physics Department Colloquium
Multiferroic Vortices and Graph Theory
Time: 4:00 pm
Place: 2241 Chamberlin Hall (coffee at 3:30 pm)
Speaker: Sang-Wook Cheong, Rutgers Center for Emergent Materials
Abstract: The fascinating concept of topological defects permeates ubiquitously our understanding of the early-stage universe, hurricanes, quantum matters such as superfluids and superconductors, and also technological materials such as liquid crystals and magnets. Large-scale spatial configurations of these topological defects have been investigated only in a limited degree. Exceptions include the cases of supercurrent vortices or liquid crystals, but they tend to exhibit either trivial or rather-irregular configurations.Hexagonal REMnO3 (RE= rare earths) with RE=Ho-Lu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerization-type structural phase transition, and this structural transition leads to three structural domains, each of which can support two directions of ferroelectric polarization. We reported that domains in h-REMnO3 meet in cloverleaf arrangements that cycle through all six domain configurations [1], Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the cycle of domain configurations is reversed. Vortices and antivortices are topological defects: even in a strong electric field they won't annihilate. Recently we have found intriguing, but seemingly irregular configurations of a zoo of topological vortices and antivortices in h-REMnO3 [2]. These configurations can be neatly analyzed in terms of graph theory and this graph theoretical analysis reflects the nature of self-organized criticality in complexity phenomena as well as the condensation and eventual annihilation processes of topological vortex-antivortex pairs.

[1] Insulating Interlocked Ferroelectric and Structural Antiphase Domain Walls in Multiferroic YMnO3, T. Choi, Y. Horibe, H. T. Yi, Y. J. Choi, Weida. Wu, and S-W. Cheong, Nature Materials 9, 253-258 (2010).
[2] Self-Organization, Condensation and Annihilation of Topological Vortices and Antivortices in a Multiferroic, S. C. Chae, Y. Horibe, D. Y. Jeong, S. Rodan, N. Lee, and S.-W. Cheong, PNAS, in print.
Host: Perkins
Poster: http://www.physics.wisc.edu/twap/posters/2011/1888.pdf
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