O.M. Stewart Colloquium

Every Monday, at 4 PM the department of Physics and Astronomy hosts the O. M. Stewart Colloquium, in rm 120, Physics Bldg. Refreshments are served starting at 3:30 PM in the Physics Library (rm 223, second floor).

Spring 2019
Date Speaker/Title/Abstract
04/08/2019 Prof. Gregory Benesh - Baylor U.
Upon Reflection Things May Not Be What They Seem!
In electronic structure calculations, large aggregates of atoms are usually approximated by model systems containing far fewer atoms—introducing artificial boundaries that do not occur in the original system.  These boundaries ordinarily produce reflected waves that interfere with outgoing solutions of the Schrödinger equation.  Depending on the degree of interference, computational results from model calculations may differ widely from the characteristics of the real physical system.  Examples of computational studies exhibiting such interference effects abound in many areas of physics.  One approach to eliminating the reflection problem is to choose Schrödinger solutions that minimally reflect at the artificial boundary of the model system.  These so-called Maximum Breaking of Time-Reversal Symmetry (MBTS) solutions come in pairs that maximally carry current in opposite directions.  In effect, by using MBTS solutions, the boundary becomes transparent or nearly-transparent to traveling waves.  The MBTS formalism and results for several model systems will be presented.
04/15/2019 Prof. Oksana Ostroverkhova - Oregon State U.
Photophysics of organic materials: from optoelectronics to entomology
Organic (opto)electronic materials have been explored in a variety of applications in electronics and photonics. They offer several advantages over traditional silicon technology, including low-cost processing, fabrication of large-area flexible devices, and widely tunable properties through functionalization of the molecules. Over the past decade, remarkable progress in the material design has been made, which led to a considerable boost in performance of organic thin-film transistors, solar cells, and other applications that rely on photophysics and/or (photo)conductive properties of the material. Nevertheless, a number of fundamental questions pertaining to light-matter interactions and charge carrier photogeneration and transport in these materials remain. In this presentation, I will briefly review the field and give examples of our efforts aiming to understand and tune exciton and charge carrier dynamics in high-performance organic materials and to develop novel, sustainable organic materials. I will also discuss how photophysics can be harnessed to manipulate wild bee populations, which can potentially be exploited in enhancing crop pollination.      
04/22/2019 Dr. Tim Charlton - Oak Ridge
FeRh Magnetic thin films: Phase boundaries to Frustration
The behavior of magnetic materials that undergo a change of magnetic state have been studied often. In particular descriptions of paramagnetic to ferromagnetic and paramagnetic to antiferromagnetic phase transformation can be found in nearly every solid state textbook. However, studies of the transition from a ferromagnet to an antiferromagnet are rare in comparison. Iron-rhodium ordered alloys are an example where this transition can be studied. With doping (Cu, Ir, Ni), one can shift the magnetization curve along the temperature scale to allow easy access to an antiferromagnetic, ferromagnetic and paramagnetic phase in the same film. Potential applications include ultra-fast switching, magnetic refrigeration, temperature and magnetic sensors.   In this presentation I will cover the effects of strain on the magnetization profile of pure FeRh films and the evolution of the ferromagnetic - antiferromagnetic phase boundary in dopant graded layers by polarized neutron reflectivity. I will also show analysis of the lateral magnetic domain structure as seen by XPEEM imaging in similar films.  Finally, I will present work using FeRhX nano-islands arranged in a square spin-ice structure in order to understand magnetic frustration away from equilibrium.
05/06/2019 Prof. Tapan Nayak - CERN Switzerland
Characterizing the Quark-Gluon Plasma at the CERN Large Hadron Collider
For only a few millionths of a second after the Big Bang, our universe consisted of a hot and dense soup of quarks and gluons, which cooled down very quickly to form protons, neutrons, and other such normal nuclear matter. The discovery and characterization of this new phase of matter called the quark-gluon plasma (QGP), require the creation of a sufficiently large and extended volume of hot and dense matter, which is possible by colliding heavy-ions at ultra-relativistic energies. The Large Hadron Collider (LHC) at CERN, commissioned in the year 2009, has collided proton-proton, proton-lead, xenon-xenon and lead-lead collisions at unprecedented energies. The ALICE (A Large Ion Collider Experiment) collaboration at the LHC has carried out a comprehensive study of the majority of particles emitted in these collisions to study the quantum chromodynamics (QCD) phase transition and to characterize the QGP phase. In the presentation, I will discuss the recreation of the baby universe in the laboratory at the LHC and the future program.
Fall 2018
Date Speaker/Title/Abstract
09/10/2018 Prof. Stefan Zollner - New Mexico State U.
Electrons and Phonons: Precision Measurements of Optical Constants
Design and fabrication of electronic and optoelectronic devices require accurate knowledge of the optical constants of all materials in the device. For fabrication, thickness and properties of device layers need to be characterized. To predict performance of optical devices, we also need to know the optical constants (absorption coefficient and refractive index) of all materials. Spectroscopic ellipsometry has been the metrology method of choice in the semiconductor industry for many years, but current applications only scratch the surface of the potential capabilities of this technique. My talk will discuss how ellipsometry can investigate some basic physics questions, especially how to study electrons and phonons in semiconductors and complex metal oxides. This work was funded, in part, by the Air Force Office of Scientific Research (FA9550-13-1-0022) and by the National Science Foundation (DMR-1505172).
09/17/2018 Prof. Emad Tajkhorshid - U. Illinois Urbana
Deciphering Biological Complexity of Membrane Proteins One Atom at a Time
Biological membranes constitute a key cellular component in all living organisms and responsible for diverse, critical cellular processes, such as signaling, transport, and cell-cell communication. Understanding the biology of the cell and physiology of multicellular organisms, therefore, depends on our ability to describe the structure, dynamics, and function of biological membranes and their components (lipids and membrane proteins) at a detailed level. While modern experimental structural biological and biophysical techniques have substantially contributed to this field, a large fraction of the molecular phenomena in biological systems are still inaccessible to experimental techniques. Computational methods, including molecular modeling and simulation, have been quite effective in complementing experiment by offering an approach that simultaneously provides the spatial and temporal resolutions needed for detailed description of cellular phenomena. In this talk, I will describe a number of recent computational studies in my lab investigating a variety of membrane-associated phenomena. In the first part, I will summarize our recent progress in employing non-equilibrium molecular dynamics simulation and advanced free energy methods to describe large-scale structural transitions in membrane transport proteins. Then I will present a number of cases in which we have focused on lipid-protein interactions and how these important effects might modulate membrane protein function and free energy landscapes associated with their function. Finally, I will present our most recent progress in cellular-scale modeling of biological membranes in their most realistic form, and advances in simulation of billion-atom molecular system. These studies have provided deep insight into the organization of biological membranes, and molecular interactions and processes within them that substantiate biological function.
09/24/2018 Dr. John Shumway - Google Inc.
Modern software development for academics
Computing has become one of the most widely transferable skills in academic research. Research groups want fast, reliable, and reproducible results, while students and scientists value career options enabled by software skills. In spite of this, fundamental ideas of modern software design have not flowed from the private sector back to academia. Academics should put software craftsmanship on par with traditional skills of writing and mathematical analysis! I will present an overview of techniques and practices to make computational research fast, reproducible, and fun. To illustrate modern computing culture, I will compare and contrast examples from my work in universities, national labs, and large and small software companies, including the skills needed for career flexibility.
10/15/2018 Prof. Ping Yu - MU
Ultra-short laser pulses
The Nobel Prize in Physics 2018 was awarded for groundbreaking inventions in the field of laser physics with one half to the “generation of high-intensity, ultra-short optical pulses”. The peak power of such pulses may be 10 TW, much more than the total electrical power produced on earth (but only during very short time ~ 30 fs).  Prof. Ping Yu of MU’s Dept. of Physics and Astronomy will present a colloquium discussing the generation of ultrashort high-intensity laser pulses and their applications in various fields.
10/22/2018 Prof. Lifan Wang - Texas A&M
FRONTIER: Faint Rapdly Evolving Transients Up to the Epoch of Reionization
I will describe a multi-cadence survey project that aims to discover and study supernovae at very early phase of explosion using the Dark Energy Camera. The same data can be stacked to reach depth deep enough to detect superluminous supernovae at redshifts approaching 6. In the talk, I will cover 1) supernovae of all Types including kilonovae, 2) numerical techniques that we have developed for wide field surveys and precision photometry, and 3) how the technique may be applied to future projects such as LSST. 
10/29/2018 Prof. Michele Pavanello - Rutgers U. Newark
Open Quantum Subsystem Dynamics in Liquids and Molecules at Surfaces
Leveraging an open-subsystem formulation of Density Functional Theory (DFT) we aim at describing periodic and molecular systems alike, including their electronic and nuclear dynamics. Subsystem DFT enables first principles simulations to approach realistic time- and length-scales, and most importantly sheds light on the dynamical behavior of complex systems. Taking subsystem DFT to the time domain allows us to inspect the electron dynamics of condensed-phase systems in real time. In liquids and interfaces, we observe all the relevant regimes proper of non-Markovian open quantum system dynamics, such as electronic energy transfer, and screening. In addition, the ab-initio modeling of system-bath interactions brought us to observe and justify the holographic time-dependent electron density theorem. Contrary to interactions between molecular (finite) systems, when molecules interact with metal or semiconductor surfaces the electron dynamics is strongly non- Markovian with dramatic repercussions to the molecule’s response to external perturbations. Metals and semiconductors typically have large polarizabilities, and even in a regime of low coupling their effect on impinging molecular species is significant – line broadening, peak shift, and intensity borrowing are observed,
11/05/2018 Dr. : Hassina Z. Bilheux - Oak Ridge National Laboratory
Neutron Imaging Capabilities and Applications at the Oak Ridge National Laboratory’s Spallation Neutron Source and High Flux Isotope Reactor
Historically, neutron imaging has been performed at reactor sources that offer a high flux of thermal and cold neutrons. At these facilities, attenuation-based neutron radiography and computed tomography have contributed to a broad range of scientific applications such as in energy storage, materials science and engineering, geosciences, plant physiology, geosciences, biology and archeology. We have installed a neutron imaging facility called CG-1D at the High Flux Isotope Reactor (HFIR) that is capable of measuring at spatial resolution ranging from 25 to 100 mm, and time resolution ranging from ms (for cyclic motions) to min. Three detectors are available at the CG-1D beamline: an ANDOR DW936 charge couple device (CCD), a ANDOR Zyla scientific complementary metal oxide semiconductor (sCMOS) detector, and a micro-channel plate (MCP) detector.  6LiF/ZnS scintillators of thickness varying from 50 to 200 mm are  being used at this facility. ORNL is building a state-of-the-art neutron imaging facility named VENUS at the Spallation Neutron Source (SNS). A pulsed source enables the collection of wavelength-dependent radiographs. In crystalline structures, narrow dips or abrupt edges in the pixel intensity are measured at precise wavelengths specified by Bragg’s law; this technique is called Bragg-edge imaging. These variations in the transmission signal can be readily utilized to study the microstructure inside a crystalline structure.  Another technique, called resonance imaging, measures the isotopic-dependent absorption of neutrons at epithermal energies. These two techniques are currently being developed at the SNS in preparation of the VENUS beamline. This seminar will highlight some of the recent scientific research performed at both neutron sources.
11/12/2018 Prof. Nicholas Suntzeff - Texas A&M
CANCELLED - Supernova Cosmology: Thirty Two Years of Watching Stars Blow Up
THIS COLLOQUIUM IS CANCELLED!   Starting in 1986, Mark Phillips, Mario Hamuy, and I began the study of the properties of nearby supernovae, and were the first to produce light curves based on CCD data. With Jose Maza, in 1989, we began the concentrated study of nearby supernovae called the Calán/Tololo Survey, which led to discoveries including the establishment of Type Ia supernovae as standardizable candles, the deeper understanding of reddening and temperature effects in light curves and spectra, and, with the HST calibration of distances to nearby host galaxies of these SNe, the modern value of the Hubble constant based on the quiet Hubble flow defined by supernovae. In 1994, Brian Schmidt and I founded the High-Z Supernova Team utilizing the Calán/Tololo results and MLCS techniques developed by Adam Riess. The image subtraction software was developed by Schmidt and later Tonry. These techniques underlie the discovery by both the HZT and the Supernova Cosmology Project of Saul Perlmutter, et al., (who developed independent software) of the apparent accelerated expansion of the Universe. All these discoveries rest on the rickety photometric system astronomers have organically developed over the last 60 years. With the improvement in the fundamental calibration system led by HST astronomers, and a reanalysis of astronomical photometric techniques by Stubbs and Tonry, we now see the results of supernova cosmology are limited by the systematic errors in how we do photometry. We founded the Carnegie Supernova Project to create a new and precisely calibrated set of nearby supernovae to dig into these systematic effects and to anchor the acceleration results. In this talk, I will present the background of supernova cosmology and my lifelong quest to measure q0.
12/03/2018 Prof. Benne Holwerda - U. Louisville
A Search for High-Redshift Galaxies And An Accidental Galactic Survey of Dwarf Stars with Hubble
Spring 2018
Date Speaker/Title/Abstract
01/22/2018 Prof. Alberto Albesa, Universidad Nacional de San Luis, Argentina
Computational simulations applied to the study of surface phenomena
02/05/2018 Prof. Pavel Lukashev, U. Northern Iowa
Half-metallicity in Heusler alloys: recent developments and prospects
In recent years, a search for materials exhibiting robust half-metallicity at ambient conditions has been one of the most technologically appealing developments in condensed matter physics and materials science. A half-metal is a material that behaves as an insulator for one spin channel and as a conductor for the opposite spin channel. Such materials are said to be 100% spin-polarized, and are particularly appealing in spintronics – an emerging technology utilizing a spin degree of freedom in electronic devices. Most of the materials reported to exhibit half-metallicity or near half-metallicity belong to the class of Heusler alloys. It has been also demonstrated in certain oxides (CrO2 and Fe3O4), manganites (La0.7Sr0.3MnO3), chalcogenides (EuN), etc. However, for practical applications Heusler alloys are especially appealing, as in general they demonstrate higher Curie temperature compared to most of the other reported half-metals. I will present my recent theoretical work on structural, magnetic, and electronic properties of these materials, and will compare theoretical results with experimental findings. In particular, I will discuss various physical mechanisms, such as atomic disorder, chemical substitution, mechanical strain, and reduced (thin film) geometry, which affect the degree of spin-polarization in potentially half-metallic compounds. 
02/12/2018 Dr. Valeria Lauter - Oak Ridge National Laboratory
Inferring the Magnetic Nanostructures for Spintronics with Polarized Neutron Scattering
02/19/2018 Reserved for candidate search
02/26/2018 Reserved for candidate search
03/12/2018 Prof. Chad Risko, U. Kentucky
Noncovalent Interactions in the Design of Organic Semiconducting Materials – A Theoretical Chemistry Perspective
Organic semiconductors (OSC), derived from π-conjugated molecules or polymers, are leading an evolution of the electronics industry towards ultra-flexible and low-power (opto)electronics. Though they have drawn the attention of materials scientists for a few decades, due to the potential to modulate material (opto)electronic properties through well-established synthetic chemistry methods, OSC design remains highly Edisonian as there is limited knowledge of the intimate relationships that connect chemical composition and molecular architecture, materials processing, and the solid-state packing arrangements that determine OSC performance. We seek to address these connections through the development and application of multiscale, theoretical materials chemistry approaches that build upon principles from organic and physical chemistry, condensed matter physics, and materials science. In this presentation, we will focus on how these models can reveal the striking influence of seemingly modest changes in chemical structure on the solid-state packing of OSC active layers and resulting characteristics of importance to charge-carrier transport. The physicochemical insight developed through these investigations is beginning to refine and offer novel design paradigms essential to the development of next generation organic semiconducting active layers, and is opening new pathways for in silico materials development.
04/09/2018 Prof. Andrew Wray, New York University
The birth of new particles from structure and disorder at a topological insulator surface
Since the discovery of the first materials with intrinsic topological electronic order one decade ago, there has been an ongoing cascade of progress in the identification of new particles, states of matter, and physical phenomena enabled by topological principles. Topologically ordered materials exhibit a phenomenon called “bulk-boundary correspondence”, whereby the quantum topology causes new exotic electronic states (termed quasiparticles) to manifest wherever the electronic structure is disrupted, such as at material interfaces, structural defects, or superconducting vortices. These exotic quasiparticles set new ground rules for the behavior of physical matter, and are at the heart of some of the most exciting recent proposals for next generation technologies. I will talk about some of my group’s explorations into the interplay between quantum topology and the nanoscale crystalline structure, based on experimental techniques that resolve single electrons (ARPES, STM). These studies have revealed new species of electronic state, including: interlinked light-like quasiparticles; quasiparticles that move along atomic step edges as if they were perfect wires; and a state that behaves like a free particle but lacks short-range translational symmetry, and thus breaks the most basic rule upon which quasiparticles are usually mathematically defined.


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