Forum Schedule Spring 2020

Fridays 3:45pm - 4:45pm BPB-217

Date Speaker Topic (click down-arrow to see abstract)
Jan 24

Jan 31

Feb 7 Miaofang Chi
Oak Ridge National Laboratory host: Qiang Zhu
Understanding interfaces via probing ions and electrons in a STEM

In spite of various physical and chemical properties, materials’ function is defined at the level of electrons and atoms. Electron microscopy has experienced significant technical developments in the past decades and now is powerful in revealing the atomic arrangements and identify chemical species of single atoms. This capability is especially crucial in studying and designing functional nanofeatures, such as interfaces, grain boundaries and point defects in energy and quantum materials, where these nanofeatures often introduce exotic chemical and physical properties that are inaccessible in their bulk counterparts. Very recently, new microscopy techniques, especially four-dimensional scanning transmission electron microscopy (4D-STEM) imaging, has opened a new era of characterizations in a STEM where not only single atoms can be imaged, but also the electron distributions at the sub-Å scale in real space. In this talk, I will first demonstrate the applications of the state-of-art and in situ STEM imaging for battery materials, by giving couple of examples on revealing ion transport and stability at the interfaces of solid electrolytes; then I will introduce 4D-STEM and differential phase contrast (DPC) imaging techniques, focusing on their capabilities in revealing electron distributions in materials. Examples of imaging electron distributions in electrides will be given.

Miaofang Chi is a senior staff scientist at the Center for Nanophase Materials Sciences (CNMS) at Oak Ridge National Laboratory (ORNL). She received her Ph.D. in Materials Science and Engineering from University of California, Davis in 2008. Her primary research interest lies in the advancements and applications of novel electron microscopy techniques in energy and nanotechnology research, especially in the understanding of interfacial charge transfer behavior in energy storage and nanoelectronics systems. She was awarded the Burton Metal by the Microscopy Society of America (2016). She received the ORNL Director’s Award for Outstanding Individual Accomplishment in Science and Technology (2015) and the ORNL’s Early Career Research Award (2015). Miaofang is the author and co-author of more than 150 peer-reviewed journal articles. She was named to the Clarivate’s 2018 list of Highly Cited Researchers.

Feb 14 Gregory Herczeg
The Kavli Institute for Astronomy
and Astrophysics, Peking University
host: Zhaohuan Zhu
From protostars to adolescence: A tour of young stellar systems

While the stages in the formation of stellar systems are now well charted, uncertainties in the initial conditions and evolution lead to stellar systems with a diverse array of architectures. In this talk I will discuss the major stages in the evolution of young stellar objects, starting from the young protostars and ending in stars that have dispersed all circumstellar material. At each step I will describe insights into some of the relevant processes that are being obtained from ongoing observational programs. For protostars, we are pursuing the first long-term monitoring program in the sub-mm to establish the role of accretion variability during the main phase of stellar growth. The next stage, protoplanetary disks, is now being revolutionized by exquisite ALMA images of substructures, which point to the presence of hidden planets. Finally, Gaia observations of young stars that have shed their disk promise to reveal the recent star formation in our local neighborhood, although this will require improved measurements of stellar properties.

Feb 21 Erin Kara
host: Daniel Proga
Reverberation mapping black hole accretion discs

Accreting supermassive black holes can produce more electromagnetic and kinetic luminosities than the combined stellar luminosity of an entire galaxy. Most of the power output from an Active Galactic Nucleus is released close to the black hole, and therefore studying the inner accretion flow is essential for understanding how black holes grow and how they affect their surrounding environments. In this talk, I will present a new way of probing these environments, through X-ray reverberation mapping, which allows us to map the gas falling on to black holes on microparsec scales and measure the effects of strongly curved spacetime close to the event horizon. I will give an overview of the field and present new results from the NICER observatory of unprecedented reverberation measurements in accreting black hole X-ray binaries.

Feb 28

Mar 6 Alexander Stephan
host: Rebecca Martin
Fates of Exoplanets in Aging Binary Systems

Exoplanets have been observed around a variety of host stars, at all stages of stellar evolution, including main-sequence, sub-giant and red giant branch stages. Additionally, white dwarf pollution signatures may indicate the presence of planetary systems in a large fraction of white dwarf systems. Moreover, since the majority of stars are in binary configurations, stellar evolution for these systems can have large implications on the planetary dynamics. I will show that the post-main sequence evolution of these systems leads to a variety of interesting phenomena, such as the formation of Hot Jupiters around evolved stars, the consumption of planets by stars, and the pollution of white dwarfs with heavy elements.

Mar 13 Gary Ferland
University of Kentucky
host: Daniel Proga
Quasar emission lines– a six decade-old grand computational/spectroscopic challenge

Quasars are the most luminous objects in the universe, so bright that we can get good spectra out to high redshift, 90% across the universe. They have strong emission lines that are formed in high-density clouds exposed to ionizing radiation. The challenge is to figure out how to use them as signposts for conditions in the early universe. I will outline the spectroscopic and computational challenges posed by their spectra, the big unanswered questions, and what we can do about them.

Mar 20 Spring Break

Mar 27

Apr 3

Apr 10 Brian Metzger
Columbia University
host: Bing Zhang

Apr 17

Apr 24 Joshua Pepper
Lehigh University
host: Jason Steffen

May 1 Ilse Cleeves
University of Virginia
host: Zhaohuan Zhu

May 8 Study Week

May 15 Patrick Kilan
Los Alamos National Laboratory
host: Daniel Proga
Numerical Plasma Physics

We have a fairly good and complete model of plasma physics for more than 80 years now. Nevertheless there are still many phenomena in plasma physics that are not fully understood. To make progress most research nowadays relies on computer simulations. But there is no single simulation method that is clearly the best in all cases. This talk will give an overview over the commonly (and not so commonly) used methods that have been developed and will discuss their upsides and limitations. I will point out connection between different simulation methods and gaps in the landscape of possible approaches.

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