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This Week at Physics

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Events on Thursday, January 31st, 2013

R. G. Herb Condensed Matter Seminar
Electronic Transport in Bi2Se3 Thin Films in the Topological Insulator Regime
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Dohun Kim, University of Maryland
Abstract: The 3D topological insulators (TIs) have an insulating bulk but metallic surface states stemming from band inversion due to strong spin-orbit interaction, whose existence is guaranteed by the topology of the band structure of the insulator. Like graphene, the STI surface state generically has a Dirac electronic spectrum with massless electrons and a vanishing bandgap at a Dirac point. In this talk, I will discuss experiments on the TI material Bi2Se3, which has a single topological Dirac surface state. Field effect transistors consisting of thin (5-17 nm) Bi2Se3 are fabricated by mechanical exfoliation of single crystals, and electrochemical and/or chemical gating methods are used to move the Fermi energy into the bulk bandgap, revealing the ambipolar gapless nature of transport in the Bi2Se3 surface states [1]. The minimum conductivity of the topological surface state is understood within the self-consistent theory of Dirac electrons in the presence of charged impurities. The intrinsic finite-temperature resistivity of the topological surface state due to electron-acoustic phonon scattering is measured to be 60 times larger than that of graphene largely due to the smaller Fermi and sound velocities in Bi2Se3 [2], which will have implications for topological electronic devices operating at room temperature. I will also discuss about our recent observation of 2D weak anti-localization (WAL) behavior in the low field magneto transport, which stems from topological surface states. By investigating gate-tuned WAL behaviors, I will show that WAL in TI regime is extraordinarily sensitive to sub-meV coupling between top and bottom topological surfaces, and interplay of phase coherence time and inter surface tunneling time results interesting crossovers from coupled single channel to decoupled multichannel coherent transports.

[1] D. Kim et al., Nature Phys. 8, 460 (2012).
[2] D. Kim et al., Phys. Rev. Lett. 109, 166801 (2012).
Host: Eriksson
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Astronomy Colloquium
Unambigious tests of general relativity with Xray Spectroscopy of AGN
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Guido Risaliti, Harvard Smithsonian Cfa/Observatory of Florence
Abstract: <br>
<br>
Active Galactic Nuclei (AGN) are optimal laboratories for tests of general relativity (GR) in the &quot;strong field&quot; regime. In particular their X-ray emission comes from a few gravitational radii from the horizon event of the supermassive black hole, and is strongly affected by GR effects. However, testing GR has been difficult so far because of a strong degeneracy in the X-ray spectra of AGN between the (relativistically distorted) intrinsic emission and the complex absorption/reflection components due to the circumnuclear medium. I will review the main diagnostic methods developed in the past few years to investigate this issue, focusing on time-resolved spectral analysis of variable sources, and I will discuss the impact in this field of the recently launched hard X-ray telescope NuSTAR. I will show how the availability of high quality broad-band X-ray spectra, and a time-resolved spectral analysis can remove the main systematic uncertainties in GR tests, provide precise measurements of black hole spins, and reveal the structure of the circumnuclear X-ray absorber.
Host: Prof Elena D'onghia
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