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This Week at Physics

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Events on Thursday, December 12th, 2013

R. G. Herb Condensed Matter Seminar
Topological Kondo Insulators
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Victor Galitski, University of Maryland
Abstract: In this talk I will review recent theoretical work on a new class of topological material systems - topological Kondo insulators, which appear as a result of interplay between strong correlations and spin-orbit interactions. I will start with introducing the by now standard theory of topological band insulators and explain the Fu-Kane method to calculate the Z2 topological index for time-reversal-invariant band structures in three dimensions. The method will be used to show that hybridization between the conduction electrons and localized f-electrons in certain heavy fermion compounds gives rise to interaction-induced topological insulating behavior. A mean field theory of these Kondo topological insulators will be derived. I will also discuss recent experimental results, which have conclusively confirmed our predictions in the Samarium hexaboride compound, where the long-standing puzzle of the residual low-temperature conductivity has been shown to originate from topological surface states. This material system represents the first true topological insulator observed experimentally with low-temperature transport dominated by the surface and essentially no conduction in the bulk. In conclusion, I will mention our ongoing theory work, which focuses on very unusual non-linear transport properties of Samarium hexaboride devices, which mimic neuron-like behavior in biological systems.
Host: Vavilov
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Newborn Pulsars as sources of Ultrahigh Energy Cosmic Rays
Time: 2:30 pm
Place: 5280 Chamberlin Hall
Speaker: Ke Fang, U. Chicago
Abstract: The workings of the most energetic astrophysical accelerators in the Universe are encoded in the origin of ultrahigh energy cosmic rays (UHECRs). Current observations by the Auger Observatory, the largest UHECR observatory, show a spectrum that agrees with an extragalactic origin, as well as an interesting transition in chemical composition from light element to heavier element as energy increases. Candidate sources range from young neutron stars to gamma-ray bursts and events in active galaxies. In this talk, we will discuss newborn pulsars as the sources of ultrahigh energy cosmic rays. We will show that a newborn pulsar model naturally injects heavier elements and can fit the observed spectrum once propagation in the supernova remnant is taken into account. With the proper injection abundances, integrated cosmic rays from the extragalactic pulsar population can match observation in all aspects - energy spectrum, chemical composition, and anisotropy. We will also examine the fingerprints of their Galactic counterparts on cosmic ray spectrum . Lastly, we will discuss the multi-messenger smoking gun of this scenario - the detectability of high energy neutrinos from pulsars and magnetars.
Host: Albrecht Karle
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