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This Week at Physics

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Events During the Week of April 21st through April 28th, 2019

Monday, April 22nd, 2019

Plasma Physics (Physics/ECE/NE 922) Seminar
Diagnosing impurity transport in the scrape-off layer of DIII-D using outer-midplane collector probes with isotopically enriched tungsten tracer particles
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Professor David C. Donovan, University of Tennessee-Knoxville (UTK)
Abstract: The impurity transport chain begins at the plasma facing component (PFC) target, connects through the edge (Scrape-Off Layer (SOL) and Divertor) plasma, and finally contaminates the core plasma. Understanding the nature of impurity transport in the SOL is therefore of crucial importance to control core contamination; however, the SOL link in the transport chain is the most weakly understood due to diagnostic limitations. CPs provide the unique capability to sample the impurity content of the SOL and their use is greatly enabled by high sensitivity material characterization techniques and interpretive modeling tools. Two isotopically distinct sources of W were installed on the lower divertor for the DIII-D MRC, which allowed the W impurities collected by the mid-plane CPs to be traced back to the poloidal location on the divertor from which they came. The CPs were combined with W source spectroscopy to determine a leakage proxy for impurities escaping the divertor and reaching the SOL. Results will be presented demonstrating that impurity leakage from the outer strike point increases nearly linearly with power across the separatrix. Experiments varying the ELM frequency identified that leakage from the far-target region is reduced significantly as ELM size decreases. The DIVIMP-OEDGE-WALLDYN code suite was utilized as an interpretive modelling tool for the edge plasma region. Experimental results have been obtained that are consistent with computational models hypothesizing an accumulation of impurities in the crown region of the SOL created by ion temperature gradients. This unique combination of impurity exposure capabilities, SOL diagnostic coverage, ex-situ material characterization, and interpretive modeling tools have provided a potent framework with which to better elucidate the SOL link in the impurity transport chain.

*Work supported by US DOE under DE-SC0016318, DE-SC0019256, DE-AC05-00OR22725, DE-FG02-07ER54917, DE-FC02-04ER54698, DE-AC04-94AL85000.
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Physics Department Colloquium
Special Colloquium
Photoinduced reaction mechanisms in molecular systems probed with elemental specificity via ultrafast X-ray spectroscopy
Time: 3:30 pm
Place: 2103 Chamberlin Hall
Speaker: Nils Huse, Hamburg University
Abstract: Understanding the interplay of structural, electronic and spin degrees of freedom is paramount for mechanistic insights into how matter transforms upon external stimuli such as light. The elemental specificity of X-ray spectroscopy provides unique complimentary information to vibrational and electronic spectroscopy or non-resonant scattering techniques when studying reaction mechanisms in chemical and materials science. This can be especially useful for metal atoms and their nearest neighbors in transition-metal complexes [1,2] or so-called heteroatoms in functional groups of organic molecules [3,4] where specific atomic sites are of particular importance for the physical and chemical properties of molecular or solid state systems. I will introduce basic concepts of X-ray absorption spectroscopy and RIXS, the X-ray analogue to resonant Raman scattering, as methods to probe electronic structure on ultrafast time scales before presenting two examples of photo-induced reactions to show how X-ray spectroscopy of ligand- and heteroatoms can (i) unravel the role of ligand-field states in transition-metal complexes and (ii) identify transient excited states and new species in sulfur-containing molecules.

[1] B. Van Kuiken et al., J. Phys. Chem. Lett. 7, 465 (2016)
[2] A. A. Cordones et al., Nat. Comm. 9, 1989 (2018)
[3] M. Ochmann et al., J. Am. Chem. Soc. 139, 4789 (2017)
[4] M. Ochmann et al., J. Am. Chem. Soc. 140, 6554 (2018)
Host: Matt Herndon
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Tuesday, April 23rd, 2019

Chaos & Complex Systems Seminar
Energy, evolution, and the origins of life?
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Terry Allard, Office of Naval Research and NASA
Abstract: It’s human nature to look for a deep understanding of who we are, where we came from and where we’re going. This presentation will explore a new hypothesis for the evolution of life articulated by Professor Nick Lane in his ground-breaking 2015 book, The Vital Question. We will review the timeline of emerging life on earth and the evolutionary relationships among the three Kingdoms: Bacteria, Archaea and Eurkaryotes. All living cells are powered by proton gradients across membranes and the secret of life is maintaining this disequilibrium through active proton pumps based on energy metabolism. Acceleration of energy production within Eukaryotes can explain the exponential growth in the complexity of life from single cells to whole organisms. Proton gradients across membranes at alkaline hydrothermal vents in the early oceans provide an explanation of how inorganic chemistry could drive organic chemistry; mechanisms of organic chemistry supporting life may have evolved before the emergence of the cell itself. This bioenergetic hypothesis suggests a very specific context for the emergence of life on earth that could be a critical variable for the search for complex life in the universe.

Primary Source Material

Nick Lane, The Vital Question: Energy, Evolution and the Origins of Complex Life (2015). W. W. Norton & Company, New York City.


Some Discussion Points

1) What is life?

2) What do the 3 Kingdoms of Life have in common and how do they differ?

3) What are the impacts of lateral gene transfer on linear descendance and the Tree of Life?

4) How does the chemistry of the ancient oceans compare to today’s oceans?

5) How did organic chemistry and cell structures emerge?

6) How likely is extraterrestrial life in the universe?
Host: Clint Sprott
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Wednesday, April 24th, 2019

No events scheduled

Thursday, April 25th, 2019

R. G. Herb Condensed Matter Seminar
Realistic numerical modeling of Majorana nanowires
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: John Gamble, Microsoft
Abstract: Using the Majorana zero modes of one-dimensional systems as topological qubits has recently generated considerable interest, with experimental efforts progressing rapidly, necessitating increasingly intricate qubit encoding schemes and layouts. Moving from a sketch of a complex design to a physical layout is a daunting engineering challenge, as small details of the design can have large impacts on device operation. Here, we present a computational tool chain that simulates the physics of these devices from the CAD schematics used for fabrication. By systematically varying the designs, we perform high-throughput computations to probe vast swaths of design space. Our simulations take into account the physical effects of self-consistent screening and superconductivity, while also including the detailed geometric configurations and fringing fields that are critical to device performance. Finally, we show validation comparisons with recent experiments in nanowire systems.
Host: Friesen
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Cosmology Journal Club
Low-Redshift Hydrogen Intensity Mapping
Time: 12:30 pm
Place: 4274 Chamberlin Hall
Speaker: Trevor Oxholm
Abstract: Special Session! Note change in time and location! Trevor Oxholm will be presenting his Prelim: "Low-Redshift Hydrogen Intensity Mapping"
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Astronomy Colloquium
The Central Engine and Outflows in the Nuclear Starburst of NGC 253 as Revealed by ALMA
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: Albert Bollato, University of Maryland
Abstract: Starbursts are a rare phenomenon in the present day universe, but they represent perhaps the most common mode under which stars form and galaxies grow during the z~1-2 peak of cosmic star formation activity. Feedback, in the form of galaxy-scale outflows, is thought to be one of the main forms of regulation of galaxy growth. NGC 253 is the premier local example for a nuclear starburst, and has been targeted with several ALMA observations in order to study how these processes work. I will discuss the properties of the molecular outflow, including our best constraints on the mass and outflow rate, and the properties of the outflowing molecular gas. I will also present the results of our recent 1.7-pc resolution observations, including the detection and properties of 14 compact structures with properties corresponding to massive young star clusters and super star clusters, 13 of which are so embedded that are invisible in optical and NIR observations (the 14th is a known SSC). I will also discuss the evidence for feedback and disruption on the scales of these clusters. Finally, I will present a preliminary analysis of the 0.5-pc resolution observations recently obtained.
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Friday, April 26th, 2019

Theory Seminar (High Energy/Cosmology)
Systematic expansion of tunneling rates
Time: 2:00 pm
Place: 5280 Chamberlin Hall
Speaker: Björn Garbrecht, Technical University Munich
Abstract: I will review the theory of tunneling in quantum mechanics and field theory to one-loop order. Then, I will address how to systematically go beyond this approximation in an expansion based on Green's functions. One phenomenological relevance of this is that a consistent calculation of the tunneling rate in the Standard Model and beyond requires a one-loop resummation. Finally, the standard Euclideanized calculation of tunneling rates is related to a derivation based on real-time amplitudes.
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Physics Department Colloquium
New Science Opportunities at X-ray Free Electron Lasers
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Uwe Bergmann, Stanford PULSE Institute, SLAC National Accelerator Laboratory
Abstract: Over the past century X-rays have revolutionized numerous fields of science. Starting in the 1970s powerful new synchrotrons sources have dramatically advanced the scientific use of X-rays. In the last decade new X-ray free electron lasers, such as the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory, have come to light. These coherent X-ray sources produce ultra-short pulses with a brightness that is ten billion times larger than synchrotron sources. For the first time scientist can study matter not just at the length scale of atoms and molecules, but also at the time scale of molecular motion. We will describe these machines and present some examples of recent X-ray laser research.

One example is our recent work on stimulated x-ray emission spectroscopy. Here, we have created inner shell x-ray fluorescence laser signals from manganese compounds. To achieve this exponential signal gain, we used highly focused ultrashort x-ray pump pulses from LCLS. The observed signal consisted of very short x-ray pulses up to 10^6 times stronger than normal x-ray fluorescence. Remarkably, these x-ray pulses preserved the small differences in fluorescence energy, caused by the different chemical surrounding of the manganese ions. We describe the phenomena and how we plan to apply this and similar approaches to learn more about transition metal systems and their function in catalysis, chemical, and materials science.
Host: Mark Eriksson
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