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This Week at Physics

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Events During the Week of September 22nd through September 29th, 2013

Monday, September 23rd, 2013

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:30 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 (lzhang263@wisc.edu)
Host: Peter Timbie
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Tuesday, September 24th, 2013

Chaos & Complex Systems Seminar
Hydrogen energy levels in n dimensions via group theory
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Adam Bincer, UW Department of Physics
Abstract: My talk is a generalization to n dimensions of a brilliant group-theoretical treatment of the hydrogen energy levels by Pauli in 1926. To start I define the concept of a group and discuss cyclic groups as a simple example. Then the rotation group in two dimensions is introduced to get us started on Lie groups. From there it is just a short hop to rotations in n dimensions. Along the way I introduce the idea of matrices - hopefully all this mathematics will be introduced gently enough so as not to turn the non mathematicians in the audience completely off. I then turn to the non-relativistic hydrogen atom in n dimensions and show how it is governed by the rotation group in (n+1) dimensions. I conclude with some anecdotes about Pauli.
Host: Clint Sprott
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Atomic Physics Seminar
Coherent Photoassociation in a Strontium BEC
Time: 2:00 pm
Place: 5310 Chamberlin Hall
Speaker: Tom Killian, Rice University
Abstract: The divalent electronic structure of alkaline-earth metal atoms such as strontium gives rise to metastable, excited triplet levels and narrow intercombination transitions. These have been exploited for optical clocks, powerful laser cooling techniques that are critical for achieving quantum degeneracy in strontium, and proposals for quantum information architectures and studies of novel magnetism. Metastable excited electronic states also give rise to narrow photoassociation (PA) transitions that differ in many ways from traditional PA with broad, electric-dipole-allowed transitions. In this talk, we will describe narrow-line PA near the ^1S_0-^3P_1 intercombination transition in an ^88Sr Bose-Einstein condensate. This results in coherent PA and Rabi oscillations between atomic and molecular condensates. Transient shifts and broadenings of the excitation spectrum are clearly seen at short times, and they create an asymmetric excitation profile that only displays Rabi oscillations for blue detuning from resonance. Coherent PA is analogous to molecule formation with a magnetic Feshbach resonance.
Host: Saffman
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Wednesday, September 25th, 2013

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Cosmic Ray Diffusion in a Dynamic Milky Way: recovering the observed B/C ratio in a spiral-armed cosmic ray propagation model
Time: 2:30 pm
Place: 4274 Chamberlin Hall
Speaker: David Benyamin, Racah Institute of Physics, Hebrew University of Jerusalem
Abstract: We develop a fully three dimensional numerical code describing the diffusion of cosmic rays in the Milky Way. It includes the nuclear spallation chain up to Oxygen, and allows the study of various cosmic ray properties, such as the CR age, grammage traversed, and the ratio between secondary and primary particles. This code enables us to explore a model in which a large fraction of the cosmic ray acceleration takes place in the vicinity of galactic spiral arms and that these spiral arms are dynamic.

We show that the effect of having dynamic spiral arms is to limit the age of cosmic rays at low energies. This is because at low energies the time since the last spiral arm passage governs the Cosmic Ray (CR) age, and not diffusion. Using the model, the observed spectral dependence of the secondary to primary ratio is recovered without requiring any further assumptions such as a galactic wind, re-acceleration or various assumptions on the diffusivity. In particular, we obtain a secondary to primary ratio which increases with energy below about 1 GeV.
Host: Paolo Desiati
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Thursday, September 26th, 2013

R. G. Herb Condensed Matter Seminar
Cavity-mediated near-critical dissipative dynamics of a driven condensate
Time: 10:00 am
Place: Chamberlin 5310
Speaker: Dr. Manas Kulkarni, Princeton University
Abstract: We investigate the near-critical dynamics of atomic density fluctuations in the non-equilibrium self-organization transition of an optically driven quantum gas coupled to a single mode of a cavity. In this system cavity-mediated long-range interactions between atoms, tunable by the drive strength, lead to softening of an excitation mode recently observed in experiments. This phenomenon has previously been studied within a two-mode approximation for the collective motional degrees of freedom of the atomic condensate which results in an effective open-system Dicke model. Here, including the full spectrum of atomic modes we find a finite lifetime for a roton-like mode in the Bogoliubov excitation spectrum that is strongly pump-dependent. The corresponding decay rate and critical exponents for the phase-transition are calculated explaining the non-monotonic pump-dependent atomic damping rate observed in recent experiments. We compute the near-critical behavior of the intra-cavity field fluctuations, that has been previously shown to be enhanced with respect to the equilibrium Dicke model in a two-mode approximation. We highlight the role of the finite size of the system in the suppression of it below the expectations of the open Dicke model.
Host: Maxim Vavilov
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Astronomy Colloquium
"Cosmic magnetogenesis: From spontaneously emitted aperiodic turbulent to ordered equipartition fields"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Reinhard Schlickeiser, Ruhr-University Bochum, Germany
Nonmagnetized fully-ionized plasmas spontaneously emit aperiodic turbulent magnetic field fluctuations. Its fluctuation intensities are dominated by the contribution from a recently found collective, damped mode, which modifies the earlier estimate of the total magnetic field strength to $|delta B|=24beta _e^{1/4}(gn_em_ec^2)^{1/2}$ G in the case of no collisional damping, where $g$ denotes the plasma parameter, $beta _e$ the thermal electron velocity in units of $c$ and $n_e$ the electron density. Accounting for simultaneous viscous damping reduces the estimate to $|delta B|=2305g(n_em_ec^2)^{1/2}$ G. For the unmagnetized intergalactic medium, immediately after the reionization onset, the field strengths from this mechanism are about $6.8cdot 10^{-13}$ G for no collisional damping and $1.5cdot 10^{-16}$ G for viscous damping. Maximum spatial scales of $10^{15}$ cm of the emitted aperiodic fluctuations are possible. These guaranteed magnetic fields in the form of randomly distributed fluctuations, produced by the spontaneous emission of the isotropic, thermal IGM plasma, may serve as seed fields for possible amplification by later possible plasma instabilities from anisotropic plasma particle distributions functions, MHD instabilities and/or the MHD dynamo process. Because of the high turbulent plasma beta, the seed fields are tied passively to the highly conducting IGM plasma as frozen-in magnetic fluxes, and therefore are subject to subsequent hydrodynamical shear or compression of the IGM medium from the shock waves of the supernova explosions of the first stars at the end of their lifetimes, or from supersonic stellar and galactic winds.
Host: Prof Alexander Lazarian
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Friday, September 27th, 2013

Theory Seminar (High Energy/Cosmology)
Triangle Anomaly in Quark-Gluon Plasma : Chiral Magnetic Wave
Time: 2:00 pm
Place: 5280 Chamberlin Hall
Speaker: Ho-Ung Yee, UIC/RIKEN-BNL
Abstract: Although the effects of triangle anomaly (chiral anomaly) in the chiral effective theory at low temperature are completely accounted for by the Wess-Zumino-Witten action, the physics of triangle anomaly at finite temperature, deconfined phase of QCD is richer and depends more on the dynamics of the theory. We discuss recently discovered new hydrodynamic effects originating from triangle anomaly, Chiral Magnetic Effect and Chiral Magnetic Wave, from several different theoretical viewpoints.
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Physics Department Colloquium
Topological Phases in Correlated Materials
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Yong-Baek Kim, University of Toronto
Abstract: Recently there have been significant theoretical and experimental efforts to understand and identify the so-called topological phases of matter in interacting electron systems. These topological phases may be characterized by different kinds of topological properties such as non-trivial edge/surface states and/or unusual elementary excitations in the bulk or surface. Notable examples include quantum spin liquids, topological insulators, and other closely related phases. One of the main challenges is to come up with theoretical criteria that can be used to identify or predict correlated materials that hold promise for the emergence of such topological phases. We discuss recent theoretical and experimental developments in this direction, along with a brief introduction to some of the proposed topological phases. In particular, we focus on correlated materials with strong spin-orbit coupling and/or near a metal-insulator transition.
Host: Perkins
Poster: http://www.physics.wisc.edu/twap/posters/2013/2992.pdf
Video: http://www.physics.wisc.edu/vod/2013/09/27.html
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2013/2013-09-23.pdf

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