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This Week at Physics

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Events During the Week of April 7th through April 14th, 2019

Monday, April 8th, 2019

Plasma Physics (Physics/ECE/NE 922) Seminar
Observation of nonlinear coupling between Kelvin-Helmholtz and drift wave instability in IMPED
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Prabal Chattopadhyay, Institute for Plasma Research (India)
Abstract: The unique control features of Inverse Mirror Plasma Experimental Device (IMPED)[1] enables production of magnetized plasmas with variable radial density, potential and electron temperature profiles over a wide range. The radial density and potential profiles are tailored to simultaneously excite Kelvin-Helmholtz and drift wave instability. These instabilities are observed to interact nonlinearly with each other leading to the formation of side bands. Bispectral analysis has been used to experimentally confirm the nonlinear coupling. The side bands are usually asymmetric in nature. However, the extent of asymmetry, i.e. the ratio of the power of the left to the right side band is controlled experimentally, which occasionally leads to symmetric side bands. The method of excitation and control of these instabilities and the probable mechanism of power distribution in side bands is presented.
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Tuesday, April 9th, 2019

Chaos & Complex Systems Seminar
In the beginning: children with disabilities in American policy, 1912-1960
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Walter Schalick, UW Department of Orthopedics and Rehabilitation
Abstract: I offer a precis of some of my work on kids with disabilities in the US, stressing scientific, clinical and governmental > policy innovations (more fun than it sounds) and their transformational effect on adults with disabilities. Most of the data comes from archives across the country.
Host: Clint Sprott
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Wednesday, April 10th, 2019

Atomic Physics Seminar
Few and many-body dynamics of Rydberg excitations in a lattice
Time: 10:00 am
Place: 5280 Chamberlin Hall
Speaker: Dr. David Petrosyan , Institute of Electronic Structure and Lasers, FORTH, Heraklion, Crete, Greece
Abstract: Strong, long-range, resonant and dispersive dipole-dipole interactions between the atoms in high-lying Rydberg states makes them uniquely suited to simulate and study various spin-lattice models. In this seminar, I will discuss two proposals [1,2] to implement spin chains with interesting properties using Rydberg atoms in a lattice.
I will show that an XXZ spin model with strong nearest-neighbor interactions and tunable long-range hopping of excitations can be realized by a regular array of laser driven atoms, with an excited Rydberg state representing the spin-up state and a Rydberg-dressed ground state corresponding to the spin-down state [1]. This scheme permits the observation of coherent quantum dynamics of spin excitations - magnons, their scattering and exotic interaction-bound states.
I will next describe a lattice of Rydberg superatoms - collections of atoms in microtraps that can each accommodate at most one Rydberg excitation. The coupling of superatoms to the laser radiation is collectively enhanced and they can represent mesoscopic, strongly polarizable spins. We studied a regular array of such effective spins driven by a laser field tuned to compensate the interaction-induced level shifts between the neighboring superatoms [2]. After the initial transient with resonantly facilitated excitation of large clusters of superatoms, the system relaxes to the steady state having nearly universal excitation density of 2/3. This state is characterized by highly-nontrivial equilibrium dynamics of quasi-particles – excitation holes in the lattice of Rydberg excited superatoms.

[1] F. Letscher, D. Petrosyan, Phys. Rev. A 97, 043415 (2018)
[2] F. Letscher, D. Petrosyan, M. Fleischhauer, New J. Phys. 19, 113014 (2017)
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Thursday, April 11th, 2019

Cosmology Journal Club
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
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 Ross Cawthon (cawthon@wisc.edu) and Santanu Das (sdas33@wisc.edu).
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
High-energy emissions from neutron star mergers
Time: 2:00 pm
Place: 4274 Chamberlin Hall
Speaker: Shigeo Kimura, PennState
Abstract: Last year, LIGO-VIRGO collaborations reported detection of the first neutron star merger event, GW170817, which accompanied with observations of electromagnetic counterparts from radio to gamma rays. High-energy gamma rays and neutrinos were not observed. However, the mergers of neutron stars are expected to produce these high-energy particles. Relativistic jets are expected to be launched when the neutron stars merge, which can be a source of high-energy neutrinos. Also, the central remnant object after the merger event, either a black hole or a neutron star, can produce high-energy photons weeks to months after the merger. In addition, the neutron star mergers produce massive and fast ejecta, which can be a source of Galactic high-energy cosmic rays, analogous to supernova remnants. In this talk, I will discuss these high-energy processes and prospects for multi-messenger detections related to the neutron star mergers .
Host: Francis Halzen
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Astronomy Colloquium
"Molecular clouds and star formation in dwarf irregular galaxies"
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: Deidre Hunter, Lowell Observatory
Abstract: Dwarf irregular galaxies are tiny compared to spirals, but they are the most numerous galaxy type in the universe. The conditions for star formation in dwarfs are different from those in spirals, and thus they give us an opportunity to challenge our ideas about star formation and galaxy evolution. I will discuss observations on the structure and formation of molecular clouds and issues of star formation in these little galaxies.
Host: Jay Gallagher
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Friday, April 12th, 2019

No events scheduled

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