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This Week at Physics

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Events on Thursday, October 23rd, 2014

R. G. Herb Condensed Matter Seminar
New Topological Excitations and Melting Transitions in Quantum Hall Effect
Time: 10:00 am
Place: Chamberlin 5310
Speaker: Yuli Lyanda-Geller, Purdue University
Abstract: Topology and symmetry define states of matter and their responses to external forces. How solids melt and become fluids, or how insulators become conductors is often controlled by excitations rather than by the ground state of systems. Non-trivial topology of excitations can alter the responses. Topological excitations are notoriously difficult to predict since they cannot be obtained as a perturbation of the ground state. In this talk I will report the discovery of a new type of topological excitations that arise in two-dimensional electron systems in a magnetic field. We investigate Landau level filling factors between integer and half-integer, which exhibit the re-entrant integer quantum Hall effect (RIQHE) with vanishing longitudinal resistance and the Hall resistance quantized to a nearest integer at lowest temperatures. I will show that charge excitations in the RIQHE regime are topologically non-trivial finite size textures of electron density with charge-dependent symmetry. These topological textures explain unusual strain dependence of resistivity. At low temperatures, the textures form a crystal, whose melting leads to metal-insulator transition.
Host: Vavilov
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Future Computing
Time: 2:30 pm
Place: 5280 Chamberlin
Speaker: Carl Anderson, IBM Fellow
Abstract: Computing has been driven by cost reduction for the last 70 years. In the next 10 years the contribution to cost reduction by hardware technology will lessen because of economic constraints and physical limits. The reduction in costs offered by automation and cloud computing will be discussed. The “Born in the Cloud” and open source programming development models are significantly reducing the cost of application development. Mobile computing and API’s are radically changing how information technology is consumed and developed. The direction of information technology is now being driven more by new business ideas than new technology ideas. The traditional Specmark metrics are being replaced by cost of use metric such as $/VM Hour.
Host: Jim Lawler
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Astronomy Colloquium
"Single white dwarfs as Progenitors of Type Ia Supernovae?"
Time: 3:25 pm
Place: 4421 Sterling Hall
Speaker: Professor Marina Orio, UW Madison Astronomy Dept
Abstract: The "normally" accepted paradigm for type Ia supernovae (SNe Ia) involves a massive CO white dwarf, or two white dwarfs, in a binary system. However, more and more evidence is being gathered, showing two observational facts: a) A very large fraction of SNe Ia must explode on sub-Chandrasekhar mass white dwarfs, b) The SN Ia rate depends on the star formation rate. I will show how these observations can be explained with a simple model,in which a single white dwarfs triggers explosive pycnonuclear reactions involving impurities of hydrogen or helium. An explosion may occur evenin the initial post-AGB cooling phase for very massive (m>1.2 M(sol) white dwarfs, or much later, just at the onset of the Debye cooling for white dwarfs in the 0.8-1.35 M(sol). Is this the the new road to take to fully understand the SNe Ia? And are these powerful explosions really useful standard candles for cosmology?
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Graduate Introductory Seminar
Astrophysics
Time: 4:30 pm
Place: 5280 Chamberlin Hall
Speaker: McCammon, Timbie
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