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This Week at Physics

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Events on Thursday, September 12th, 2019

R. G. Herb Condensed Matter Seminar
Transport properties of a compensated metal: the Lorentz ratio and the absence of mass renormalization near a Pomeranchuk quantum critical point
Time: 11:00 am
Place: 5310 Chamberlin Hall
Speaker: Songci Li , UW-Madison
Abstract: A violation of the Wiedemann-Franz law in a metal can be quantified by comparing the Lorentz ratio, $L=kapparho/T$. We obtain the Lorentz ratio of a clean compensated metal with intercarrier interaction as the dominant scattering mechanism by solving exactly the system of coupled integral Boltzmann equations. The Lorentz ratio is shown to assume a particular simple form in the forward-scattering limit: $L/L_0=overline{Theta^2}/2$, where $Theta$ is the scattering angle. In this limit, $L/L_0$ can be arbitrarily small. We discuss how a strong downward violation of the Wiedemann-Franz law in a type-II Weyl semimetal WP$_2$ can be explained within our model. In the second part of the talk, we discuss the role of mass renormalization in electron transport of a compensated metal near a QCP. According to a naive interpretation of the Drude formula, as electrons get heavier near a QCP, their electrical and thermal conductivities decrease. However, this picture has never been supported by an actual calculation. We employ a model case of a compensated metal near a Pomeranchuk-type QCP. The advantage of this model is that it allows one to treat electrical and thermal conductivities on the same footing, without invoking umklapp scattering or any other channels of momentum relaxation which are extraneous to the electron system. By solving the kinetic equations, we obtain explicit results for the electrical and thermal conductivities of a two-band compensated metal. We show that mass renormalization factors cancel out with the $Z$ factors, which renormalize the scattering probability, so that all the transport quantities contain the bare rather than renormalized electron masses. We also demonstrate how the same conclusion can be drawn by diagrammatically calculating the optical conductivity.
Host: Alex Levchenko
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Cosmology Journal Club
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
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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Astronomy Colloquium
The Yin and Yan of Slowly-Pulasting B Stars: Asteroseismology and Angular Momentum Redistribution
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: Professor Richard Townsend, UW Madison Astronomy Department
Abstract: During their main-sequence evolution, almost all B-type stars will pass through a phase where they are unstable toward oscillation in one or more global internal gravity waves ('g modes'). The g modes, driven by iron and nickel opacity in the stars' outer envelopes, generate surface temperature and velocity changes with periodicities on the order of days.

In the 'Yin' part of my talk, I'll discuss how time-series spectroscopy and photometry of these `slowly-pulsating B' (SPB) stars can be leveraged into asteroseismology --- probing the stars'interiors by careful analysis of their oscillation spectra. I'll highlight in particular how the Kepler mission, together with the MESA stellar evolution code and my GYRE stellar oscillation code, has allowed novel constraints to be established on the internal rotation and mixing physics of SPB stars.

I'll then pivot to the 'Yang' part of my talk. Although we typically regard stellar oscillations as passive tracers of stellar structure, they can also modify this structure. I'll present recent work with Jacqueline Goldstein and Ellen Zweibel, exploring angular momentum redistribution by g modes. Modeling this process in SPB stars, we find that significant modification of internal rotation profiles can occur on timescales as short as centuries. This suggests that the g modes can impact the stars' life trajectories, a possibility that's been hitherto ignored in stellar evolution calculations.
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