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This Week at Physics

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Events During the Week of October 2nd through October 9th, 2011

Monday, October 3rd, 2011

Plasma Physics (Physics/ECE/NE 922) Seminar
TBD
Time: 12:05 pm
Place: 2317 Engineering Hall
Speaker: Greg Howes, University of Iowa
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R. G. Herb Condensed Matter Seminar
Fe-based superconductors at high magnetic fields
Time: 1:30 pm
Place: 5310 Chamberlin
Speaker: Alex Gurevich, Old Dominion University
Abstract: Low carrier densities and short coherence lengths in the recently discovered semi-metallic Fe-based superconductors (FBS) can result in exotic behaviors at strong magnetic fields due to the interplay of multiband superconductivity, unconventional pairing symmetry and the Zeeman and orbital pairbreaking. I this talk I will give an overview of these effects, particularly the anomalous temperature dependencies of the upper critical field Hc2(T) which often extrapolate to Hc2 > 100 Tesla at low temperatures for some FBS. The materials features of FBS and the multiband s+/- pairing symmetry with the sign change of the order parameter on different sheets of the Fermi surface can significantly increase Hc2(T) and facilitate the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) transition to the state with a spatially modulated order parameter. Small shifts of the chemical potential upon doping of FBS can produce new small pockets of the Fermi surface, triggering the FFLO instability due to the Lifshitz transition. I will also discuss the effect of disorder, particularly magnetic defects produced by α-particle irradiation.
Host: Rzchowski
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Condensed Matter Theory Group Seminar
Strong coupling theory of quantum critical points in heavy fermion compounds
Time: 4:30 pm
Place: 5310 Chamberlin
Speaker: Peter Wölfle, Karlsruhe Institute of Technology, Germany
Abstract: Following a brief general introduction into quantum phase transitions the case of antiferromagnetic transitions in heavy fermion metals is discussed. It is shown that in certain cases the effective mass of fermionic quasiparticles diverges at the critical point. Using an extension of Landau quasiparticle theory the coupling of the critical behavior in the fermionic and bosonic sector is shown to lead to a new type of scaling behavior with unusual critical exponents. The theory accounts well for the up to now unexplained critical behavior of YbRh2Si2.
Host: Perkins
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Tuesday, October 4th, 2011

Chaos & Complex Systems Seminar
Topological chaos
Time: 12:05 pm
Place: 4274 Chamberlin
Speaker: Jean-Luc Thiffeault, UW Department of Mathematics
Abstract: Topological chaos is a type of chaotic behavior that is `forced' by the motion of obstacles in some domain. I will review two topological approaches, with applications in particular to stirring and mixing in fluid dynamics. The first involves constructing systems such that the fluid motion is topologically complex, usually by imposing a specific motion of rods. I will discuss optimization strategies that can be implemented. The second is diagnostic, where flow characteristics are deduced from observations of periodic or random orbits and their topological properties.
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Theory Seminar (High Energy/Cosmology)
A global perspective on string inflation
Time: 4:00 pm
Place: Chamberlin 5280
Speaker: Ted van der Aalst, Leiden University
Abstract: Although inflation is a coarse phenomenon in classical gravity, it is remarkably difficult to describe it in (worldsheet) string theory due to a sensitive dependence on global information of the theory. This sensitivity provides an opportunity to probe stringy phenomena by cosmological observations, but only if the model is fully rooted into string theory. We will discuss some of the requirements for obtaining string inflation both in a supergravity context as well as from a worldsheet perspective. At the worldsheet level, inflation indeed poses coarse but generic conditions on the worldsheet theory, manifesting itself via non-perturbative effects.
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Wednesday, October 5th, 2011

Department Meeting
Canceled
Time: 12:15 pm
Place: 5310 Chamberlin Hall
Department Meeting Canceled.
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Thursday, October 6th, 2011

R. G. Herb Condensed Matter Seminar
Superconducting gap symmetry in novel superconductors: insights from quasiparticle interference
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Ilya Eremin, Ruhr-University Bochum, Germany
Abstract: Coherence factors are a hallmark of superconductivity as a pair-condensation phenomenon. When electrons pair, quasi-particles develop an acute phase sensitivity to different types of scattering potential, described by the appearance of coherence factors in the scattering amplitudes. While the effects of phase-sensitive coherence factors are well established in isotropic superconductors, they are much harder to detect in their anisotropic counterparts, such as high-Tc cuprates, iron-based superconductors, or heavy-fermion superconductors. One of the recently developed technique to determine the phase structure of the superconducting order parameter is scanning tunneling microscopy (STM) which determines the dispersion of quasiparticle states from the quasiparticle interference (QPI) patterns induced by impurity scattering. Such experiments, performed in an external magnetic field, offer the capability of probing the phase of the superconducting order parameter by detecting a field enhancement of the sign preserving scattering that results from the sensitivity of QPI to the coherence factors associated with impurity scattering. In my talk I will review the experimental and theoretical results for layered cuprates and present calculations of the QPI patterns in iron-based superconductors and heavy fermion superconductor, CeCoIn5 where the symmetries of the superconducting gaps are still actively debated.
Host: Chubukov
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Going to the ends of the earth to glimpse the beginnings of time: Observing the Big Bang with the BICEP Telescope at the South Pole
Time: 2:30 pm
Place: 4274 Chamberlin Hall
Speaker: Brian Keating, UCSD Department of Physics
Abstract: The Background Imager of Cosmic Extragalactic Polarization (BICEP) experiment is the first cosmic microwave background (CMB) polarimeter designed to measure the "B-mode" polarization of the CMB, hypothesized to originate during the Inflationary epoch. Beginning in 2006 BICEP observed 3% of the sky from our observatory at the Amundsen-Scott South Pole Research Station in Antarctica. In this colloquium I will present our initial results and discuss the unique design features of BICEP which led to the first meaningful limits on the energy scale of Inflation to come from CMB polarization. Soon after BICEP's initial results were released, a publication (Xia, Li & Zhang, 2009), claimed a first-detection of parity-violating "cosmic birefringence" effects using publicly available BICEP data. I will discuss the challenges of polarimetry at the few parts per billion level and explain why systematic effects are particularly pernicious for probes of cosmic parity violation. I will conclude by discussing how BICEP and its successor, BICEP2, currently in its second observing season at the South Pole will constrain Inflationary cosmology and future measurements of cosmic birefringence.
Host: Peter Timbie
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Graduate Introductory Seminar
Condensed Matter Theory Seminar
Time: 5:30 pm
Place: 2223 Chamberlin Hall
Speaker: Chubukov, Coppersmith, Joynt, Perkins, Vavilov
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Friday, October 7th, 2011

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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Physics Department Colloquium
No Colloquium
Poster: http://www.physics.wisc.edu/twap/posters/2011/2300.pdf


"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2011/2011-10-03.pdf

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