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Events on Thursday, December 1st, 2022

R. G. Herb Condensed Matter Seminar
Imaging the Breaking of Electrostatic Dams in Graphene for Ballistic and Viscous Fluids
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Zachary Krebs , UW-Madison
Abstract: Under special conditions the charge carriers in a material can behave as a viscous fluid. In this work, we investigate such behavior by using scanning tunneling potentiometry to probe the nm-scale flow dynamics of electron fluids in graphene as they pass through constrictions defined by smooth and tunable in-plane p-n junction barriers. We observe that as the sample temperature and channel widths are increased, the electronic fluid flow undergoes a Knudsen-to-Gurzhi transition from ballistic to viscous flow that is characterized by a channel conductance that exceeds the ballistic limit, and suppressed charge accumulation against the barriers. Our results are well-modeled by finite-element simulations of 2D viscous current flow, and they illustrate how Fermi liquid flow evolves with carrier density, channel width, and temperature.
Host: Alex Levchenko
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Preliminary Exam
Exploration of problems in space physics using the Magnetospheric Multiscale Mission
Time: 10:00 am
Place: B343 Sterling or
Speaker: Jack Schroeder, Physics Graduate Student
Abstract: The main topic of discussion will be on recent work on magnetic reconnection in the Earth’s magnetotail. Models for collisionless magnetic reconnection in near-Earth space are distinctly characterized as 2D or 3D. In 2D kinetic models, the frozen-in law for the electron fluid is usually broken by laminar dynamics involving structures set by the electron orbit size, while in 3D models the width of the electron diffusion region is broadened by turbulent effects. We present an analysis of in situ spacecraft observations from the Earth's magnetotail of a fortuitous encounter with an active reconnection region, mapping the observations onto a 2D spatial domain. While the event likely was perturbed by low-frequency 3D dynamics, the structure of the electron diffusion region remains consistent with results from a 2D kinetic simulation. As such, the event represents a unique validation of 2D kinetic, and laminar reconnection models.

Additionally, future work regarding magnetic pumping at Earth’s bow shock will be discussed.
Host: Jan Egedal
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
The Changing Landscape of Astrophysical Searches and Indirect Detection of Dark Matter
Time: 2:30 pm
Place: CH 4274
Speaker: Eric Charles, SLAC
Abstract: Ten years ago, astrophysical searches and indirect detection of dark matter largely meant “searches for WIMP annihilation signatures in specific targets such as Dwarf Spheroidal galaxies or the Galactic Center with the Fermi Large Area Telescope”. Since then the landscape has expanded dramatically; the techniques we use, the places we look, the sort of information that we hope to extract from our searches, all of these have evolved as we have explored the sky in new ways with the advent of new observing facilities such as GAIA, DES, IceCube, and will continue to evolve as JWST, Euclid, Roman and Rubin revolutionize the way we see the sky. In this talk I will describe those changes, the motivations behind them, and the implications for how we need to work together going forward to make the best use of the facilities that will be coming on-line
Host: Ke Fang
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Astronomy Colloquium
Why do massive stars have "inflated" cores?
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Dr. Evan Henry Anders, Northwestern University
Abstract: Stars with masses greater than about 1.1 M_sun have turbulent convection in their cores. Standard stellar evolution models fail to reproduce many observations, but models and observations can be brought into agreement by "inflating" the core with excess mixing beyond the boundary of the convection zone. In this colloquium, I will present a review of observations of excess mixing in the cores of massive stars. I will discuss how excess mixing affects stellar evolution and the populations of stars and compact objects which are being characterized by space-based missions like Gaia and ground-based gravitational wave interferometers like LIGO. I will then discuss different forms of convective boundary mixing from a fluid dynamics perspective, talking about three processes which likely occur in stellar interiors. I will discuss one or two sets of simulations which we are using to shed light on this tricky problem in modern stellar astrophysics.
Host: Ke Zhang
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