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Events on Thursday, March 29th, 2012

R. G. Herb Condensed Matter Seminar
Atomic-Layer Engineering and Topological insulators
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Seongshik Oh, Dept. of Physics & Astronomy, Rutgers University
Abstract: Atomic-layer engineering was first made possible in III-V semiconductors in 1980s by MBE (Molecular Beam Epitaxy), and later in complex oxides in 1990s by MBE and in 2000s by PLD (Pulsed Laser Deposition). The challenges of atomic-layer engineering in complex-oxides compared to those in semiconductors will be discussed. Then I will describe how we apply this technique to a new family of materials called topological insulators (TIs). Topological insulators are predicted to have metallic surface states with polarized spins while inside of the material is insulating. These unique properties pose TIs as promising candidates for spintronics and quantum computation. However, material problems in the current-generation TIs are a major hurdle to implementing real TI devices. I will discuss how we utilize the atomic-engineering schemes to overcome this hurdle and navigate through the vastly unexplored territory of topological insulators.
Host: Rzchowski
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Probing TeV scale physics in ultra-cold neutron decays
Time: 2:30 pm
Place: 4274 Chamberlin
Speaker: Rajan Gupta, Los Alamos National Laboratory
Abstract: This talk will describe the possibility of probing scalar and tensor interactions
arising from new physics at the TeV scale at the two experiments, UCNb
and UCNB, being developed at the ultra-cold neutron source at Los Alamos.
I will show that the largest uncertainty is connecting theory to experiments
(with measurements at the 10^{-3} level), comes from estimates of the
scalar and tensor charges of a nucleon. I will then discuss the status of our lattice
QCD calculations of these matrix elements and plans for the future. As summary, a
discussion of current bounds on these novel interactions will be presented.
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Astronomy Colloquium
"X-ray Spectroscopy of Massive Stars: Constraints on Stellar Wind Mass-Loss Rates, Clumping, and Shock Physics"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: David Cohen, Swarthmore College
Abstract: Hot, luminous, massive stars are strong sources of soft X-ray emission, despite their general lack of surface magnetic fields and the associated dynamo activity and MHD processes that heat a corona and explain the X-ray emission of solar-type stars. Since massive stars' surprising discovery as X-ray sources in the late 1970s, a consensus has emerged that the X-rays are produced in their strong radiation-driven winds. I will show how high-resolution X-ray spectroscopy carried out over the past decade with the Chandra and XMM-Newton satellite observatories has confirmed the basic wind-shock scenario of X-ray production in massive stars and enabled the testing of theoretical models by providing quantitative constraints on the temperature, spatial distribution, and kinematics of the X-ray emitting plasma embedded in the stellar wind. I will also discuss how absorption signatures in the X-ray spectra can be used to measure the mass-loss rates of these winds, and present some initial results that indicate lower mass-loss rates than have been traditionally assumed. When combined with observations in the optical and radio, these X-ray absorption measurements also provide information about the clumping properties of massive star winds.
Host: Richard Townsend
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