Forum Schedule Fall 2017

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

Date Speaker Topic (click down-arrow to see abstract)
Sep 1 Physics&Astronomy Department Meeting

Sep 8

Sep 15 John Tse
University of Saskatchewan
host:. Qiang Zhu
Structure, Structural Evolution and Superconductivity of High Pressure Hydrogen-rich Alloys

Pressure has an enormous effect in altering the structure, physical and chemical properties of matters. In general, for elemental metals, compression led to loosely bound valence electrons redistributed into the interstitial vacancies and often resulted in novel open 2-D and 3D structures. Incidentally, when mixed with H2, which has an electronegativity similar to group 13 and 14 elements, charge transfer Zintl-Klemen type compounds can be formed. This phenomenon helps to explain the structural trend in hydrogen-rich alloys predicted by First Principle methods. Under suitable conditions, these alloys may even become superconductors and, in some cases, with very high critical temperature (Tc). Analysis of the functional derivative of Eliashberg spectral functions show that efficient electron-phonon coupling over the entire vibrational spectrum can be achieved on crystal structures when the stretch and bend vibrations of weakly linked hydrogen network atoms are strongly mixed.

Sep 22 Hanno Rein
University of Toronto
host: Jason Steffen
Formation and Stability of Planetary Systems

The diverse population of extrasolar planets keeps challenging theories of planet formation. Multi-planetary systems are of particular interest as their dynamical architectures allow us to constrain an otherwise unobservable formation phase.

I'll show how a simple stability requirement together with machine learning tools can be used to constrain orbital parameters of planetary systems such as HL-Tau and Trappist-1. Some of the dynamical properties we find in the these systems can be explained by a turbulent protoplanetary disk and stochastic planet migration. Saturn's rings can be thought of as a small scale version and test bed of the early Solar System. I'll show evidence that stochastic migration can be directly observed in moonlets around Saturn.

I'll finally talk about some of the numerical challenges when running accurate long term simulations of planetary systems. Even though we are solving differential equations that have been known since Newton's time, several breakthroughs were made only very recently with the help of clever numerical algorithms. Among these is the discovery that one percent of all realization of the Solar System lead to collisions between planets within the life-time of the Sun.

Sep 29

Oct 6 Simon Kimber
Oak Ridge National Lab
host:. Ashkan Salamat
High Energy Insights Into Materials Science

3rd generation synchrotrons and spallation neutron sources provide high-energy beams of x- rays and neutrons. In the last ten years, these sources have been increasingly applied to materials science problems. This is due to the highly penetrating power of the radiation, unique atomic contrast, and access to reciprocal space far beyond that available with laboratory sources. In this presentation I shall describe recent advances in sample environment and technique, which have given users access to mechanochemical synthesis [1]. This has provided unique insights into ZIF production [2], short-lived co-crystal intermediates [3] and even non- equilibrium phase formation [4]. I will next show how advances in data processing have allowed the development of pair distribution function (PDF) contrast computed tomography [5]. This developing technique allows the mapping and characterisation of nano-particle dispersion in supported catalysis bodies [6]. Finally, I will discuss PDF scattering in the liquid state [7], and prospects for ‘crystallography beyond the crystal’, in systems from polyoxometalates to pharmaceuticals. The presentation will conclude with an update on new techniques under development at ORNL, including dynamic PDF measurements and advanced high pressure sample environment. Finally, an update on the newly commissioned POWGEN neutron powder diffractometer will be given. This is currently the highest resolution such machine in North America.

Oct 13 Shyue Ping Ong
UC San Diego
host: Qiang Zhu
Deep Learning Quantum Mechanical Calculations to Design Materials

Powered by methodological breakthroughs and computing advances, electronic structure methods have today become an indispensable toolkit in the materials designer’s arsenal. In this talk, I will discuss two emerging trends that holds the promise to continue to push the envelope in computational design of materials. The first trend is the development of robust software and data frameworks for the automatic generation, storage and analysis of materials data sets. The second is the advent of reliable central materials data repositories, such as the Materials Project, which provides the research community with efficient access to large quantities of property information that can be mined for trends or new materials. I will show how we have leveraged on these new tools to accelerate discovery and design in energy and structural materials as well as our efforts in contributing back to the community through further tool or data development.

Oct 20 Canceled

Oct 27 Nevada Day Recess

Nov 3 David Nesvorny
Southwest Research Institute
host: Jason Steffen
Dynamical Effects of the Planetary Instability/Migration on the Asteroid and Kuiper Belts

The solar system has changed dramatically since its birth, and so did our understanding of it. A considerable research effort has been invested in the past decade in an attempt to reconstruct the solar system history, including the earliest stages some 4.5 billion years ago. The results indicate how several processes, such as planetary migration and dynamical instabilities, acted to relax the orbital spacing of the outer planets, and provided the needed perturbation to explain the present planetary orbits that are not precisely circular and coplanar. Here I will highlight the dynamical effects of planetary instability and migration on the orbital structure of the asteroid and Kuiper belts.

Nov 10 Veterans Day Recess

Nov 17 Alexander Shtukenberg
NYU
hosts: Qiang Zhu & Ashkan Salamat
Twisted crystals

Modern reviews on crystal growth and defects in crystals overlook the fact that a significant portion of materials can spontaneously grow as single crystals with twisted and bent morphologies and curved crystal lattices. Twisted crystals can be found among all types of materials crystallizing in any crystallographic symmetry from all types of growth media. Their size spans over more than six orders of magnitude ranging from nm to dm. Here are illustrated most important features of twisted crystals as well as an analysis of mechanisms that are responsible for this mysterious phenomenon. A special attention is paid to evolution of the twist intensity as crystal grows and increases in size.

Nov 24 Thanksgiving Day Recess

Dec 1 Fred Adams
University of Michigan
host: Zhaohuan Zhu
Fine-Tuning Constraints on Stellar Operations

Motivated by the possible existence of other universes, with different values for the fundamental constants, this talk considers stars and stellar structure with different values for the fundamental constants of nature. Focusing on the fine structure and gravitational constants, we first enforce the following constraints: [A] long-lived stable nuclear burning stars exist, [B] planetary surfaces are hot enough to support chemistry, [C] stellar lifetimes are long enough to allow biological evolution, [D] planets are massive enough to maintain atmospheres, [E] planets are small enough to remain non-degenerate, [F] planets are massive enough to support complex biospheres, [G] planets are less massive than stars, and [H] stars are less massive than galaxies. The parameter space that satisfies these constraints is relatively large: viable universes can exist when the structure constants vary by several orders of magnitude. Next we consider a number of other fine-tuning issues, including the triple alpha fine-tuning problem for carbon production, nucleosynthesis in universes without stable deuterium, and structure formation in universes with varying amplitudes for the primordial density fluctuations. In all of these scenarios, the basic parameters of physics and cosmology can vary over wide ranges and still allow the universe to operate.

Dec 8 Ho-kwang Mao
Geophysical Lab of the Carnegie Institution for Science
host: Qiang Zhu
NOTE: Moved to BPB-102
Solids, Liquids, and Gases Under High Pressure

Pressure has long been recognized as a fundamental thermodynamic variable, but its application was previously limited by the available pressure vessels and probes. The development of multi-megabar pressure vessels and a battery of associated in-laboratory and synchrotron techniques at the turn of the century have opened a vast new window of opportunities. With the addition of the pressure dimension, we are facing a new world with an order of magnitude more materials to be discovered than all that have been explored at ambient pressure. Pressure drastically and categorically alters all elastic, electronic, magnetic, structural, and chemical properties, and pushes materials across conventional barriers between insulators and superconductors, amorphous and crystalline solids, ionic and covalent compounds, vigorously reactive and inert chemicals, etc. In the process, it reveals surprising high-pressure physics and chemistry and creates novel materials. I will describe the principles and methodology used to reach ultrahigh static pressure, the in situ probes, the physical phenomena to be investigated, the long-pursued goals, the surprising discoveries, and the vast potential opportunities. Examples include the recent advances and surprising findings in high-pressure research of hydrogen, oxygen, iron, and carbon. Overall, this review demonstrates that high-pressure research is a new dimension in physics, chemistry, Earth and materials sciences.

Dec 15 Alex Lazarian
University of Wisconsin - Madison
host: Bing Zhang
New Way of Tracing Astrophysical Magnetic Fields: Velocity Gradients

I shall discuss how the theory of MHD turbulence predicts that gradients of velocity are aligned perpendicular to magnetic field and how this fact opens a radically new way to study magnetic fields using spectroscopic data. I will demonstrate how well the new technique works comparing the velocity gradient predictions with the results obtained by Planck for diffuse media and BLASTPOL for molecular clouds.

Dec 20 Miroslav Popovic
University of California, Berkeley NOTE: Wednesday Dec 20 @ 3:00PM BPB-217
Characterization of the Oxide Layers Formation on Fe-Cr-Al Substrate, and Strain Induction in Substrate, In Exposure to Liquid Pb45Bi55

Liquid lead-bismuth eutectic (LBE) is a powerful heat transfer fluid, but ​ ​ it suffers from a high solubility of many elements from structural materials (solids) above 550 C. A proper concentration of oxygen dissolved in LBE leads to formation of passivizing surface oxide structures that maintain diffusion control over the elements dissolution (leakage). Previous studies analyzed how the various parameters affect the structure of oxide layers. Here we studied the influence of Al content, O concentration in liquid, at 700 and 800 C, on structure of oxides developed. Oxide phases were analyzed by TEM, Raman and XPS, and a complex structure of layers was detected, consisting of various Al-, Cr- Fe- and Ti- oxides. However, breaking and spalling of oxide layers in cooling to the room temperature is another issue, which has been studied here by synchrotron Laue-XRD. Two possible causes of the strain detected in near-interface bulk have been considered: (1) thermal constriction mismatch (between oxides and substrate) and (2) internal oxidation (mostly of Al). Calculated thermal-constriction-induced strain predicts the values lower than experimentally measured values, however a theoretical model of internal oxidation gives a better agreement, thus indicating it as a main cause of strain. Raman shifts also indicate a strain in oxide phase atop the bulk. Based on theoretical presumptions and experimental results, models of (1) oxide layers formation and (2) strain generation have been​ ​ proposed and discussed.

Future forums: Spring 2018

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