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Events at Physics

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Events on Thursday, November 5th, 2015

R. G. Herb Condensed Matter Seminar
Detecting frequency fluctuations in mesoscopic oscillators
Time: 10:00 am
Place: Chamberlin 5310
Speaker: Yaxing Zhang, Michigan State University
Abstract: Mesoscopic oscillators, such as nanomechanical resonators or optomechanical systems, usually experience comparatively large fluctuations due to their small size. An important type of fluctuations is frequency fluctuations, that is, the oscillator eigenfrequency is subject to a random perturbation in time. For nanomechanical resonators, frequency fluctuations can come from tension and mass fluctuations, fluctuating charges in the substrate, or dispersive inter-mode coupling; for electromagnetic cavity modes, they can come from fluctuations of dielectric constant. Frequency fluctuations lead to the broadening of oscillator linewidth. However, this broadening is partly masked by the broadening due to oscillator decay, making the identification of frequency noise difficult. In this talk, I will show that the interplay of a resonant driving and frequency noise can lead to specific features in the oscillator power spectrum that allow sensitive detection and quantitive characterization of frequency noise. I will start with a generic analysis of a harmonic oscillator with frequency noise, corroborated by measurements on a carbon nanotube resonantor [1]. Then I will discuss a particular type of frequency noise that comes from dispersive inter-mode coupling based on microscopic descriptions [2].

[1] Zhang, Y., Moser, J., GĂĽttinger, J., Bachtold, A. & Dykman, M. I. Interplay of driving and frequency noise in the spectra of vibrational systems. Phys. Rev. Lett. 113, 255502 (2014).
[2] Zhang, Y. & Dykman, M. I. Spectral effects of dispersive mode coupling in driven mesoscopic systems. Phys. Rev. B 92, 165419 (2015).

Host: Vavilov
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Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:15 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 Amol Upadhye (aupadhye@wisc.edu).
Host: Amol Upadhye
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Astronomy Colloquium
Dwarf Galaxies, the Local Group, and Cosmology
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee 3:30 pm, Talk 3:45 PM
Speaker: James Bullock, UC Irvine
Abstract: The Local Group and the tiny galaxies that surround the Milky Way provide unique and detailed data sets for testing ideas in cosmology and galaxy formation. In this talk I will discuss how numerical simulations coupled with local "near-field" observations are informing our understanding of dark matter, the formation of the first galaxies, and the physical processes that act at the threshold of galaxy formation.
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