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This Week at Physics

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Events During the Week of October 7th through October 14th, 2012

Monday, October 8th, 2012

Plasma Physics (Physics/ECE/NE 922) Seminar
No seminar
Time: 12:00 pm
Place: 1310 Sterling Hall
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Condensed Matter Theory Group Seminar
Electronic properties of ultimate nanowires
Time: 4:30 pm
Place: 5310 Chamberlin
Speaker: Franz Himpsel
Abstract: Stepped semiconductor surfaces decorated with metal atoms make it possible to engineer the ultimate nanowire, a string of atoms connected by a single wave function. Such atomic wires form spontaneously for many metal-semiconductor combinations and thereby provide a playground for low-dimensional electron physics [1]. Angle-resolved photoelectron spectroscopy combined with scanning tunneling microscopy/spectroscopy reveals a variety of unexpected phenomena, such as atomically-precise ribbons of graphitic silicon and spin-polarized Fermi surfaces caused by the Rashba effect.

The broken bond orbitals are so localized at the step edges that they become fully spin-polarized, similar to isolated broken bonds (Pb centers). Density functional theory predicts a spin-polarized ground state which is closely coupled to the formation of a superlattice (neither can exist without the other) [2]. This resembles the situation in complex oxides (HiTc superconductors, multiferroics), where the spin, charge, and lattice degrees of freedom are strongly coupled and make it difficult to find a simple approximation of their electronic structure.

Two-photon photoemission spectroscopy [3] and scanning tunneling spectroscopy [4] provide support for spin-polarized silicon atoms by probing their characteristic minority spin state.


REFERENCES

1. I. Barke et al., Low-dimensional electron gas at semiconductor surfaces, Solid State Commun. 142, 617 (2007);
Nuri Oncel, Atomic chains on surfaces, J. Phys. Condens. Matter 20, 393001 (2008);
J. Schaefer et al., Self-organized atomic nanowires of noble metals on Ge(001): atomic structure and electronic properties, New Journal of Physics 11, 125011 (2009);
P. C. Snijders and H. H. Weitering, Electronic instabilities in self-assembled atom wires, Rev. Mod. Phys. 82, 307 (2010).
2. S. C. Erwin and F. J. Himpsel, Intrinsic magnetism at silicon surfaces, Nature Communications 1:58 (2010).
3. K. Biedermann et al., Spin-split silicon states at step edges of Si(553)-Au, Phys. Rev. B 85, 245413 (2012).
4. P. C. Snijders et al., Spectroscopic evidence for spin-polarized silicon atoms on Si(553)-Au, submitted.

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Tuesday, October 9th, 2012

Chaos & Complex Systems Seminar
A few things that we learned from Galileo, and a few things we didn't
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Jim Lattis, Director, Director, UW Space Place, UW Department of Astronomy
Abstract: There are quite a few myths and misunderstandings about Galileo's scientific career, many of them commonly propagated by scientists in the classroom. This historical talk will endeavor to examine and correct some common myths about Galileo and to highlight some of the lessons that we should take away from the events of his tumultuous life.
Host: Sprott
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Wednesday, October 10th, 2012

No events scheduled

Thursday, October 11th, 2012

R. G. Herb Condensed Matter Seminar
The Kondo exciton: a quantum quench towards strong spin-reservoir correlations
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Hakan Tureci, Princeton University
Abstract: When a quantum system is subjected to a quantum quench, its subsequent dynamics is governed by energy scales that become ever lower with increasing time: whereas the transient behavior right after the quench depends on high-energy excitations, the asymptotic long-time evolution is determined by low-lying excitations close to the final ground state. Thus, time- or frequency-resolved probes of the dynamics after a quantum quench offers insight into the nature of the system's eigenstates across the entire energy spectrum. We recently proposed [1] that such a quantum quench for the single-impurity Anderson Model can be induced by the sudden creation of an exciton in a quantum dot via optical absorption of an incident photon of definite frequency. The subsequent emergence of correlations between the spin degrees of freedom of the dot and a tunnel-coupled low-temperature Fermionic reservoir, ultimately leading to the Kondo effect, can be accurately mapped out through an optical absorption experiment. This experiment was recently carried out [2] with semiconductor quantum dots coupled to a degenerate electron gas, demonstrating experimentally for the first time the optical signature of Kondo correlations. I will discuss the theory behind the resulting lineshape that is found to unveil three very different dynamical regimes, corresponding to short, intermediate and long times after the initial excitation, which are in turn described by the three renormalization group fixed points of the Anderson Model. At low temperatures and just beyond the absorption threshold, the lineshape is dominated by a power-law singularity, with an exponent that is a universal function of magnetic field and gate voltage.

[1] H. E. Tureci et al., Phys. Rev. Lett. 106, 107402 (2011).
[2] C. Latta et al., Nature 474, 627 (2011).
Host: Vavilov
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Searches for Lorentz Violation in the Top Quark Sector
Time: 2:30 pm
Place: 4274 Chamberlin
Speaker: Denver Whittington, Indiana University
Abstract: Lorentz symmetry posits that the laws of physics are invariant regardless of the orientation and velocity of the reference frame in which they are measured. Violation of this symmetry can be quantified using the Standard-Model Extension (SME) framework, which predicts the effects that Lorentz violation would have on elementary particles and their interactions. This model predicts a dependence of the production cross section for top and antitop quark pairs on sidereal time as the orientation of the experiment changes with the rotation of the Earth. In this talk I will present the results of a search for Lorentz violation in ttI,, events using data collected with the D0 detector at the Fermilab Tevatron Collider, setting upper limits on parameters within the SME describing the possible strength of Lorentz violation in the top sector. I will also investigate the prospects for extending this analysis using the ATLAS detector at the Large Hadron Collider which, because of the higher rate of top-antitop events at that experiment, has the potential to improve the limits determined at D0.
Host: Karle
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Graduate Introductory Seminar
Condensed Matter Theory
Time: 5:45 pm
Place: 2223 Chamberlin Hall
Speaker: Chubukov, Coppersmith, Joynt, Perkins, Vavilov, UW Madison
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Friday, October 12th, 2012

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
    http://cmb.physics.wisc.edu/journal/index.html
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Le Zhang (lzhang263@wisc.edu)
Host: Peter Timbie
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Theory Seminar (High Energy/Cosmology)
Phenomenology of next generation neutrino oscillation experiments
Time: 2:00 pm
Place: 5280 Chamberlin
Speaker: Kaoru Hagiwara, KEK
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Physics Department Colloquium
Nuclear Power After Fukushima
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Michael Corradini, UW-Madison
Abstract: The Tohoku earthquake caused a tsunami, which hit the east coast of Japan, and caused a loss of all on-site and off-site power at the Fukushima Daiichi site, leaving it without any emergency power. The resultant damage to fuel, reactor and containment caused a release of radioactive materials to the region surrounding the plants. Although not directly affected, the U.S. nuclear power industry will take lessons from this accident. The American Nuclear Society (ANS) formed a special committee to examine the Fukushima Daiichi accident. The committee was charged to provide a clear and concise explanation of the accident events, health physics and accident cleanup as well as safety-related issues that emerged. The committee also evaluated actions that ANS should consider to better communicate with the public during a nuclear event. This talk will discuss the accident and the future of nuclear power after Fukushima.
Host: Himpsel
Poster: http://www.physics.wisc.edu/twap/posters/2012/2681.pdf
Video: http://www.physics.wisc.edu/vod/2012/10/12.html
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2012/2012-10-08.pdf

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