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This Week at Physics

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Events on Thursday, October 19th, 2017

R. G. Herb Condensed Matter Seminar
Cyclotron resonance in graphene: Kohn’s theorem, many-particle physics, and more
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Erik Henriksen , Washington University in St. Louis
Abstract: Cyclotron resonance—the resonant absorption of infrared light by charge carriers in a strong magnetic field– is one of the more basic measurements that can be made on semiconductors. First demonstrated on germanium in the early ‘50s, CR proved to be enormously useful in determining semiconductor band structures and, from the ‘70s, was instrumental in probing two-dimensional systems in Si and GaAs heterostructures. However, early on W. Kohn pointed out a limitation of the CR technique: in translationally invariant parabolic systems, CR is insensitive to electron-electron interactions, with the consequence that CR has been useless in investigating such remarkable phenomena as the fractional quantum Hall effect. We have performed CR measurements over the past decade that demonstrate how to evade Kohn’s theorem by breaking translational invariance, or working in graphene whose linear dispersion can be viewed as an extreme case of a non-parabolic band structure. In our most recent work we find direct evidence of many-particle physics in the cyclotron resonance of high mobility graphene. This exciting development suggests that infrared spectroscopy will provide a new window on interacting electron phenomena in graphene including (fractional) quantum Hall effects, Hofstadter’s butterfly, hydrodynamic transport, and perhaps even cavity QED.

Host: Brar
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Round Table Discussion of Recruitment and Retention of Women in the Graduate Program
Time: 2:00 pm
Place: B343 Sterling Hall Conference room
Speaker: Jenni Strabley
Abstract: Round Table Discussion of Recruitment and Retention of Women in the Graduate Program
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Astronomy Colloquium
Climate Cycles and the History of Our Solar System
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookes 3:30 PM. Talk begins at 3:45 PM
Speaker: Stephen Meyers, Department of Geoscience, UW Madison
Abstract: Quasiperiodic variations in Earth’s orbit and spin vector influence the distribution of sunlight on Earth’s surface, causing cyclic climate change on time scales >10,000 years. The geologic record preserves evidence of these astronomical-climate rhythms, which are expressed as physical, chemical and paleobiologic variability in sedimentary strata. In this seminar, I will discuss how these “fossilized” astronomical signals can be used to test hypotheses about the behavior of the Solar System, and its evolution over the past several billion years. One of the most fundamental questions that we can address with the geological archive pertains to the proposed chaotic dynamical nature of the Solar System that is predicted by numerical and analytical models of the planetary orbits. New geological data from the Cretaceous period (~86 million years ago) provides the first unambiguous confirmation of this chaotic motion. Another question that the geologic record is uniquely poised to address is the history of the Earth-Moon system, including changes in Earth-Moon separation and length of day. To address this research problem, I will present results from a new Bayesian inversion approach that uses the astronomical signals preserved in very ancient strata (> 1 billion years old) to reconstruct the Earth-Moon history, as well as the fundamental frequencies of the Solar System. A byproduct of this work is the development of high-resolution geological time scales that can be used to assess rates of Earth system processes, including climate change and biological evolution. Taken together, these examples highlight the close interlink between geology and astronomy, and how these fields can fundamentally inform each other.
Host: UW Astronomy Deparment
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