This Week at Physics

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Events During the Week of March 4th through March 11th, 2012

Monday, March 5th, 2012

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Antiparticles in the shadow of the Earth: cosmic-ray positrons with the Fermi Gamma-ray Space Telescope
Time: 11:00 am
Place: 5280 Chamberlin Hall
Speaker: Justin Vandenbroucke, SLAC
Abstract: Cosmic rays are primarily protons and other nuclei, but there is a small flux of electrons and an even smaller flux of positrons. Cosmic-ray positrons can be produced by astrophysical accelerators, by collisions of cosmic-ray protons with interstellar gas, or by dark matter. We used the Fermi Gamma-ray Space Telescope to measure the charge-separated electron and positron energy spectra. Because Fermi does not have an onboard magnet, we used the Earth's magnetic field to distinguish positrons and electrons. We confirmed the PAMELA discovery that the positron fraction is rising with energy between 10 and 100 GeV and measured the positron flux for the first time in the 100-200 GeV range. Explaining the positron excess remains an outstanding question.
Host: Halzen
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Plasma Physics (Physics/ECE/NE 922) Seminar
Using liquid metal experiments to test models of MHD turbulence
Time: 12:00 pm
Place: 2241 Chamberlin
Speaker: Mark Nornberg, UW-Madison
Abstract: Experiments using liquid metals to demonstrate MHD instabilities like the generation of magnetic field through a dynamo or magnetically induced turbulence through the Magnetorotational Instability provide a wealth of information on MHD turbulence. Recent measurements of the correlated velocity and magnetic fluctuations on the Madison Dynamo Experiment reveal that simple mean-field models, assuming homogeneous and isotropic turbulence, describe the measured transport of magnetic field rather well. Turbulence in a rapidly rotating fluid, like that of an experiment designed to study the magnetorotational instability, has a strong anisotropy however which tends to confine the motions of the fluid to two-dimensional, wave-like turbulence. I will present some ideas for experiments using a rapidly rotating liquid metal to study the effects of magnetic fields on these waves and to test mean-field models for describing turbulent transport in these flows.
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Condensed Matter Theory Group Seminar
Collective instabilities of fermions on a hexagonal lattice
Time: 4:30 pm
Place: 5310 Chamberlin
Speaker: Andrey Chubukov, UW-Madison
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Tuesday, March 6th, 2012

R. G. Herb Condensed Matter Seminar
Magnetic interactions and anisotropies in iron superconductors
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Leni Bascones, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC)
Abstract: A major breakthrough took place in 2008 with the discovery of high temperature superconductivity in iron pnictides. The basic structure of these materials are FeAs layers. In these materials superconductivity emerges when doping or applying pressure to an antiferromagnetic compound. Most iron pnictides order with (pi,0) momentum, antiferromagnetic in the x direction and ferromagnetic in the y direction. The magnetic state of iron pnictides is metallic and presents unusual magnetic moment and resistivity anisotropy [1].

The origin of magnetism in iron superconductors is still unsettled. At the heart of the debate are the itinerant (weak coupling) versus localized (strong coupling) nature of magnetism, the role played by the orbital degree of freedom and Hund's coupling and a possible nematic phase. The itinerant picture relies on the metallicity of the magnetic state and an approximate (pi,0) nesting of the Fermi surface. In the strong coupling picture localized moments interact with AF exchange constants J1 and J2 with their first and second nearest neighbors respectively. (pi,0) state can be stabilized with J1< 2 J2, but little is known on the actual value of these constants. Orbital ordering has been proposed as a leading instability and to explain the anisotropic properties.

In this talk I will present our recent calculations which try to make connection between the itinerant and localized picture. We have analyzed the magnetic interactions of iron superconductors on the basis of a five orbital model treated both within Hartree-Fock and Heisenberg approximations [2]. We show that the exchange constants depend on charge and orbital filling and on Hund's coupling with unexpected consequences on the phase diagram [2]. We have also studied the dc and optical conductivities and separated the effect of magnetism and of orbital ordering on the anisotropic properties [3].

[1] For a review see, J. Paglione and R.L. Greene, Nature Phys. 6, 645 (2010).
[2] M.J. Calderon, G. Leon, B. Valenzuela and E. Bascones, arXiv:1107.2279. E. Bascones, M.J. Calderon, B. Valenzuela, PRL 104, 227201 (2010)
[3] B. Valenzuela, E. Bascones, M.J. Calderon, PRL 105, 207202 (2010)
Host: Perkins
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Chaos & Complex Systems Seminar
Perturbation in the system: Creative network practice
Time: 12:05 pm
Place: 4274 Chamberlin
Speaker: Michelle Riel, Artist, Designer, Theorist
Abstract: In art practice, use of computers, electronics, and telecommunications began in the 1960s. What then differs in today's creative network practice? In particular, chance operations of seminal 20th century artists and exploration of systems theory by mid-century artists have become the dynamical systems of today's creatives who employ coding, physical computing, and emerging networked technologies and methods. I relate historical precursors to current work and discuss how network practices employ non-linear dynamic processes.
Host: Sprott
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Wednesday, March 7th, 2012

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, March 8th, 2012

R. G. Herb Condensed Matter Seminar
Raman scattering in molecular junctions
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Michael Galperin, University of California, San Diego
Abstract: Recent advances in experimental techniques at nanoscale, and in particular Raman scattering measurements on current-carrying single molecule junctions promises to become a superior diagnostic tool. Theoretical understanding of optical response of open molecular systems far from equilibrium is of major importance for development of molecular optoelectronic devices. Within simple models we consider intra-molecular and charge-transfer contributions to Raman spectroscopy of molecular junction. We also discuss a concept of "effective temperature", its relevance in representation of bias-induced heating, and ability of Raman measurements to provide information on the latter.
Host: Irena Knezevic & Robert Joynt
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High Energy Seminar
Higgs Results From ATLAS
Time: 3:00 pm
Place: 4274 Chamberlin
Speaker: Bruce Mellado, University of Wisconsin Madison
Abstract: The ATLAS Collaboration reported an excess of events with the
H to gammagamma,ZZ channels in December 2011. Results of the low
mass Higgs boson search with channels that were not reported
then will be reviewed. Results with the H to WW,tautau,bbar
channels and their combination with results reported earlier will
be discussed.
Host: Matthew Herndon
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Friday, March 9th, 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:
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 (
Host: Peter Timbie
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Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Physics Department Colloquium
Superconductivity from Repulsive Interactions
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Andrey Chubukov, University of Wisconsin Department of Physics
Abstract: The theory of superconductivity, for which the Nobel Prize was given in 1972, named electron-phonon interaction as a glue that overcomes Coulomb repulsion and binds fermions into pairs which then condense and super-conduct. I review recent and not so recent works aiming to understand whether a nominally repulsive Coulomb interaction can by itself give rise to a superconductivity. I will first discuss a generic scenario of the pairing by electron-electron interaction, put forward by Kohn and Luttinger back in 1965 in their attempt to explain superfluidity in 3He, and then move on to discuss modern studies of the electronic mechanisms of superconductivity in the cuprates, Fe-pnictides, and even in graphene.
Host: Department
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