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Events During the Week of December 12th through December 19th, 2010

Monday, December 13th, 2010

R. G. Herb Condensed Matter Seminar
Electronic correlations in the optical properties of the iron-based superconductors
Time: 12:00 pm
Place: 5310 Chamberlin
Speaker: Christopher Homes, Brookhaven National Lab
Abstract: The complex conductivity of the iron-arsenic superconductor BaFe1.85Co0.15As2 (Tc ≅ 25 K) and the iron-chalcogenide superconductor FeTe0.55Se0.45 (Tc ≅ 14 K) have been determined over a wide frequency range for light polarized in the planes at a variety of temperatures above and below Tc. The optical properties of the superconducting materials are compared with their metallic parent compounds, BaFe2As2 and Fe1.03Te. Despite being multiband systems, the low-frequency conductivity in the electron-doped iron-arsenic material is dominated by a single (electron) pocket, which can be described by a simple Drude model. Not surprisingly, the electron-boson coupling constant λ ≅ 1, indicating that this material is not strongly correlated. Below Tc there is a clear signature of the superconductivity in the optical conductivity and there is evidence that more than one energy scale is involved in the superconductivity. However, at low temperature (T << Tc) less than 50% of the free carriers have collapsed into the condensate indicating that this material is not in the clean limit [1]. In contrast, the low-frequency conductivity of the iron-chalcogenide compounds is non-Drude at low temperature, with λ ≅ 5 - 7 in the superconducting material, indicating a much greater degree of electronic correlation. Below Tc there is once again evidence for two energy scales in the superconductivity - this may be interpreted as either the isotropic gapping of the electron and hole pockets, or the anisotropic gapping of the electron pocket (s+/- model) [2]. Both the iron-arsenic and the iron-chalcogenide materials fall close to the universal scaling line for a BCS dirty-limit superconductor in the weak-coupling limit, as do the majority of the high-temperature copper-oxide superconductors.

[1] J. J. Tu et al., Phys. Rev. B 82, 174509 (2010).
[2] C. C. Homes et al., Phys. Rev. B 81, 180508(R) (2010); J. Phys. Chem. Solids (in press).
Host: Andrey Chubukov
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Plasma Physics (Physics/ECE/NE 922) Seminar
"The Surprising? Effect of Lithium Wall Coatings on Edge Profiles, Transport and ELM Stability in NSTX"
Time: 12:00 pm
Place: 2535 Engineering Hall
Speaker: Rajesh Maingi, Oak Ridge National Laboratory
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Tuesday, December 14th, 2010

No events scheduled

Wednesday, December 15th, 2010

Last Day of Class
Time: 8:00 am
Place:
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Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, December 16th, 2010

Astronomy Colloquium
"Star Formation and Feedback in High-Redshift Galaxies"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Kristian Finlator, UC-Santa Barbara
Abstract: The last decade has seen an explosion in observational constraints on galaxy growth at high redshifts (z&gt;2). These data offer a wealth of insight into fundamental questions such as the roles of kinetic and photoionization feedback and the relative duty cycles of smooth versus bursty star formation. Over the same period, advances in computing have enabled cosmological hydrodynamic simulations to incorporate remarkably detailed treatments for galaxy growth and feedback processes in representative cosmological volumes. Using comparisons between predictions from these models and observations of the luminosity functions, colors, metallicities, and clustering properties of young galaxies, I will show that recent observations argue strongly in favor of high-redshift star formation histories that are predominantly smoothly-rising and regulated by strong outflows. I will discuss areas that are ripe for further inquiry on both theoretical and observational fronts.<br>
Host: Prof Christi Tremonti
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Friday, December 17th, 2010

No events scheduled