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This Week at Physics

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Events During the Week of May 4th through May 11th, 2014

Monday, May 5th, 2014

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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Plasma Physics (Physics/ECE/NE 922) Seminar
HIT-SI Results
Time: 12:00 pm
Place: 2241 Chamberlin
Speaker: Brian Nelson, University of Washington, Seattle
Abstract: Steady inductive helicity injection (SIHI) current drive produces spheromaks with up to 90 kA toroidal current, and plasma current to injector current ratios of up to 3.8, in the Helicity Injected Torus-Steady Inductive (HIT-SI) experiment. Unlike most helicity injection methods, SIHI directly applies a periodic non-axisymmetric distortion, called Imposed Dynamo Current Drive (IDCD). IDCD eliminates the need for the plasma itself to produce non-axisymmetric relaxation mechanisms, and furthermore can be used to sustain a kink-stable equilibrium. HIT-SI creates and sustains spheromaks with equilibrium reconstructions (fitting to internal magnetic field measurements) that are ideally n=1 kink-stable. Furthermore, at injector frequencies greater than the inverse sound transit time, equilibrium fits to internal magnetics show pressure confinement with high beta, as well as exhibiting both an outward shift of the magnetic axis and improved toroidal symmetry. The high efficiency of IDCD sustaining stable equilibria with high beta, enables spheromak reactor designs with overnight capital costs competitive with present power-generating plants.
Host: John Sarff
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Condensed Matter Theory Group Seminar
Magnetization and current
Time: 4:30 pm
Place: 5310 Chamberlin Hall
Speaker: Artem Abanov, TAMU
Abstract: I will discuss ways to manipulate various magnetization textures by the
current. I will start by introducing magnetization, both static and dynamic. I
will show how magnetic domain walls can be moved by time dependent
resonant current. I will discuss the ways to measure different
properties of the magnetization textures in nano wires and nano dots,
and the ways to produce nontrivial textures with the current. The effects of
the topologically nontrivial textures in the magnetization on the current will
also be shown.

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Tuesday, May 6th, 2014

Chaos & Complex Systems Seminar
Twentieth Anniversary Celebration
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Robin Chapman, UW Department of Communicative Disorders
Abstract: We will celebrate the 20th anniversary of this weekly seminar with a look at the questions asked across the years and the questions that the audience wants to see answered now. What have we learned, and what's next? Bring your thoughts! The webpage contains abstracts of the twenty years of talks.
Host: Clint Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
First Light with HAWC Gamma-Ray Observatory
Time: 1:20 pm
Place: 2223 Chamberlin
Speaker: Stefan Westerhoff, UW Madison Department of Physics
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Theory Seminar (High Energy/Cosmology)
The Double-Dark Portal
Time: 4:00 pm
Place: 5280 Chamberlin Hall
Speaker: David Curtin, Stony Brook University
Abstract: In most models of the dark sector, dark matter is charged under some new symmetry to make it stable. We explore the possibility that not just dark matter, but also the force carrier connecting it to the visible sector is charged under this symmetry. This dark mediator then acts as a Double-Dark Portal. We realize this setup in the dark mediator Dark matter model (dmDM), featuring a fermionic DM candidate chi with Yukawa couplings to light scalars phi. The scalars couple to SM quarks via the operator q q phi phi /Lambda. This can lead to large direct detection signals via the 2->3 process chi N -> chi N phi if one of the scalars has mass < 10 keV. We undertake the first systematic survey of constraints on light scalars coupled to the SM via the above operator, including LHC experiments, cosmological considerations and stellar astrophysics, and find the strongest constraints come from neutron star cooling observations. We also explore the direct detection consequences of this scenario and find that a heavy ~ 100 GeV dmDM candidate fakes different ~ 10 GeV WIMPs at different experiments.
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Wednesday, May 7th, 2014

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, May 8th, 2014

R. G. Herb Condensed Matter Seminar
On the way to understanding quantum matter: The death and rebirth of quasiparticles
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Peter Wolfle
Abstract: The low energy excitations of quantum matter generally have particle-like character. Their existence allows a relatively simple and economical description of thermodynamic and transport properties of condensed matter at low temperatures. However, the stability of these quasiparticles may be destroyed by fluctuations, e.g. at a classical or quantum phase transition into an ordered state. Inside the ordered phase the quasiparticles may be reborn in different form, together with other kinds of new quasiparticles. Examples are quantum phase transitions of metals into an antiferromagnetic or into a superconducting/superfluid state. Fluctuations may be also enhanced by dimensional reduction or by frustrating interactions, leading to the appearance of new types of quasiparticles. As examples one-dimensional quantum wires and the fractional quantum Hall effect will be considered,where “holons” and “spinons” , composite fermions and fractionally charged quasiparticles can be found
Host: Perkins
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Astronomy Colloquium
The Disks of the Milky Way Galaxy
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Rosemary Wyse, The John Hopkins University
Abstract: I will discuss our current understanding of the disk(s) of the Milky Way. The vertical structure of stellar disks is determined by the relative importances of diverse physical processes, including gaseous dissipation prior to star formation, subsequent gas accretion into the disk, heating mechanisms such as interactions with transient spirals, and the mass ratios and gas content of merging systems. The radial structure reflects star-formation rates, angular momentum (re)distribution and interactions within the disk. The kinematic, chemical and age distributions of the stellar populations of present-day disks, as a function of scale-height and scale-length, provide further constraints on disk evolution. Decomposition of disks into distinct spatial components -- such as thin and thick -- is most meaningful when the spatial decomposition is accompanied by distinct stellar populations and/or different physical processes determining their properties. The most detailed information is available for stars in the Milky Way galaxy and I will demonstrate this decomposition based on recent results for the Milky Way disks.
Host: Prof John Gallagher
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Friday, May 9th, 2014

Physics Department Colloquium
“Phun” with Photoelectrons - or - What Sets the Tc in Cuprate High-Temperature Superconductors?
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Dan Dessau, University of Colorado-Boulder
Abstract: Angle-resolved photoemission spectroscopy (ARPES) has recently emerged as one of the most powerful probes of the electronic structure of a solid, directly giving the detailed energy-momentum dispersion relations (band structures), Fermi surfaces, etc. – properties that are the starting point for almost all analyses of the physical properties of a solid. Going more deeply, ARPES also has unique abilities to uncover the dynamical interaction effects or “self energies” that dominate the physical properties of correlated electron materials.

Perhaps the most famous and exotic of correlated electron materials are high-temperature cuprate superconductors, which have exotic “normal” and superconducting states, neither of which are understood. Here I show our latest results on this problem, focusing not just on the pairing energy scale (the gap Δ) but also the pair-breaking energy scale Γ. In contrast to conventional superconductors in which the superconducting transition temperature Tc is set by the pairing energy alone, I show that Tc in the cuprates is set by a crossover between the pairing and pair-breaking energy scales, each of which is strongly temperature-dependent. I then discuss how this is likely related to the strong interactions present in the normal state, with many of these interactions “undressing” as the material goes superconducting.
Host: Department of Physics
Poster: http://www.physics.wisc.edu/twap/posters/2014/3179.pdf
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2014/2014-05-05.pdf

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