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This Week at Physics

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Events on Thursday, December 8th, 2016

R. G. Herb Condensed Matter Seminar
Universal transport at the edge: Disorder, interactions, and topological protection
Time: 10:00 am
Place: Chamberlin 5310
Speaker: Matthew Foster, Rice University
Abstract: Topological insulators and superconductors provide condensed matter realizations of the holographic principle: a topological property of the bulk translates into an anomalous time-reversal symmetry at the material surface. This symmetry underlies "topological protection" of the edge or surface states. Protection from disorder effects (Anderson localization) is particularly nontrivial, because surfaces are low dimensional. While this was previously understood for noninteracting models of edge and surface states, the more complicated problem of combined disorder and interaction effects had not been addressed until recently. I will discuss two different examples of universal edge or surface transport that arise in the presence of both. First, I will consider the edge states of 2D topological insulators with Rashba spin-orbit coupling (RSOC). With RSOC, disorder induces a backscattering term in the edge theory. We have shown that transport remains perfectly ballistic in a model that incorporates this term and interactions. The solution involves a mapping to spin 1/2 moment that executes perfect adiabatic evolution in a random magnetic field. [1] Second, I will discuss the surface states of 3D topological superconductors, and explain why we predict universal surface thermal and (if conserved) spin conductivities. The solution combines various nonperturbative and/or exact methods including conformal field theory, the Finkel'stein non-linear sigma model, and numerics. [2,3] I will also show how disorder enhances topological protection for surface states of recently introduced [4] model spin-3/2 topological superconductors, which may have applications to ultracold atoms or Half-Heusler compounds.

Refs:
[1] H.-Y. Xie, H. Li, Y.-Z. Chou, and M. S. Foster, PRL 116, 086603 (2016)
[2] H.-Y. Xie, Y.-Z. Chou, and M. S. Foster, PRB 91, 024203 (2015)
[3] M. S. Foster, H.-Y. Xie, and Y.-Z. Chou, PRB 89, 155140 (2014)
[4] W. Yang, Yi Li, and C. Wu, PRL 117, 075301 (2016)
Host: Alex Levchenko
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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:href="http://cmb.physics.wisc.edu/journal/index.html">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
The Turbulent Origin of Stars
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 pm, Talk at 3:45PM
Speaker: Phillip Chang, UW Milwaukee
Abstract: Recent numerical experiments on the star formation rate on small scales find that the star formation rate is much faster than the Kennicutt-Schmidt law which well-characterizes star formation on galactic scales, and also shows that star formation accelerates with time. Motivated by these results, I will discuss a new dynamical theory of star formation in a turbulent medium that seeks to explain these observed numerical results. I will survey the various theories of star formation and their shortcomings. I will then discuss how we extend previous theories of collapse by considering turbulence as a dynamical variable and closing the fluid equations with a new form of the energy equation. The resulting theory explains these previous numerical results and makes a few predictions that have recently been numerically verified as well as some numerical surprises. I will discuss the implications of this theory on observations.
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