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).

Fall 2019
Date Speaker/Title/Abstract
09/16/2019 Prof. Erik Henriksen - Washington U
Cyclotron resonance spectroscopy of symmetry broken states in mono- and bilayer graphene
Cyclotron resonance—the resonant absorption of light by charge carriers in a strong magnetic field—is widely used to measure the effective band mass of (semi-)conducting materials. This works because the CR absorption by charge carriers in a parabolic dispersion, which is a reasonable description of most materials, is unaffected by inter-particle interactions. An intriguing corollary is that, for instance, in high-mobility GaAs heterostructures when the electronic transport shows remarkably complex behavior in the regime of the fractional quantum Hall effect, there is still only a single cyclotron resonance peak that is qualitatively little different from a low-mobility device. But: in materials with a linear dispersion such as graphene, this proscription against spectroscopy of interactions does not hold. We have built a dedicated infrared magnetospectroscopy setup for exploring the cyclotron resonance of interacting Dirac systems, and will report our progress including an exciting observation of full integer symmetry breaking of the underlying Landau levels in monolayer graphene. We will also show recent measurements in bilayer graphene and discuss plans for ’shining light’ on other correlated electron systems.
09/23/2019 Prof. Ni Ni - UCLA
Discovery of intrinsic van der Waals magnetic topological insulators
New materials are the driving force for technology innovations and our progressive understanding of condensed matter physics. In the last decade, breakthroughs have been made on topological materials. The discovery of bulk materials with non-trivial topology has led to rich new emergent phenomena, including Dirac surface state, Fermi arc surface state, chiral pumping effect, colossal photovoltaic effect, quantum anomalous Hall effect, etc. In this talk, I will present our discovery of a family of intrinsic van der Waals topological insulators. By reducing the interlayer magnetic exchange interaction, I will show that this family of materials can be tuned from antiferromagnetic to ferromagnetic, providing a superior material platform to investigate quantum anomalous Hall effect, Majorana fermions, etc. 
10/07/2019 Prof. Kwon Park - Korea Institute of Advanced Study
What is Quantum Matter?
As far as we know, quantum mechanics governs our universe and all matters therein. It is, however, not easy to feel the existence of quantum mechanics in our everyday life since quantum mechanics manifests itself usually in the microscopic world. Condensed matter physics is a branch of physics, where researchers study quantum phenomena in the macroscopic world. Among such phenomena are superconductivity, where the electric resistance is entirely zero, and the quantum Hall effect, where the electric resistance performs quantum jumps. Quantum matter is the phase of matter exhibiting macroscopic quantum phenomena. Amazingly, new classes of quantum matter can emerge when electrons become strongly correlated. High-temperature superconductors and the fractional quantum Hall states are the strongly correlated versions of the previous two examples of quantum matter. In this talk, I would like to explain the incredible world of quantum matter and its strongly correlated frontiers.
10/14/2019 Prof. Haojing Yan - MU
2019 Nobel Prize in Physics: A new understanding of the universe's structure and history
The Nobel Prize in Physics 2019 was awarded to three astrophysicists for their "contributions to our understanding of the evolution of the universe andEarth's place in the cosmos". Half of the prize went to James Peebles for his "theoretical discoveries in physical cosmology". Most notably, these include his pioneering research in revealing the origin of the cosmic microwave background (CMB) in 1970's and in establishing the cold dark matter (CDM) model of the universe in 1980's, both of which have fundamentally shaped our current view of the universe. The other half of the prize was awarded jointly to Michel Mayor and Didier Queloz for their "discovery of an exoplanet orbiting a solar-type star" in 1995, which has opened up the window to the numerous "new worlds". To date, more than 4,000 extra-terrestrial planets have been found, and the study of exoplanets has become a new, the most fast-growing branch of astronomy. This year's Laureates have given us "new perspectives on our place in the universe". This talk is to present and to explain their research.
10/21/2019 Prof. Bala Iyer - International Centre for Theoretical Sciences, Bengaluru, India
Justing Huang Special Colloquium: Not with a Whimper but a Bang: From Gravitational Wave Detection to Multi-Messenger Astronomy
The first  detection of gravitational waves from a  black hole binary in  2015 was a  breakthrough, taking a century to realize, and made possible by the coming together of a remarkable experiment and an exquisite theory complemented by the best in sophisticated data analyses, state of the art computing and  the transition to "big science". For this discovery,  Rainer Weiss, Kip Thorne and Barry Barish  received the Nobel Prize for Physics   in 2017. The   discovery of gravitational waves  from a neutron star binary in 2017  and the intense associated electromagnetic follow up heralds the launch of a  new  multi-messenger astronomy  with  its  potential to  impact  astrophysics, cosmology and fundamental physics. A week  after the announcement of the discovery in Feb 2016, LIGO-India received its in-principle approval from the Indian government.  The talk concludes with  a brief summary of the LIGO-India project, its   current status and future prospects.
10/28/2019 Prof. Wouter Hoff - Oklahoma State University
Using photoreceptors and spectroscopy to understand proteins
In many respects proteins are attractive catalysis to form the basis of renewable technology, but such applications remain limited. Similarly, understanding proteins promises a direct avenue to address a wide range of human diseases, but many medical challenges persist. This situation suggests that fundamental knowledge on proteins remains to be discovered. Two complementary approaches to gain such knowledge are to discover and employ generally applicable principles in protein science and to develop novel methods for studying proteins. In this seminar I will present how spectroscopic studies of photoreceptor proteins are contributing to both of these challenges.
11/04/2019 Prof. Efrain Rodriguez - U. Maryland
Hydrogen Bonding and Symmetry Relationships in FeSe-based superconductors
I will present our work on iron-based superconductors where hydrogen bonding plays a role in stabilizing structures that would otherwise not exist. In this lecture I will focus on the layered iron-based superconductors and the intercalated phases such as (Li1-xFexOH)FeCh, [Na1-xFex(OH)2]FeCh, and [Li(C2H8N2)y]FeCh where Ch is S and Se. New physics can be uncovered through such synthetic methods since it can for example place a ferrimagnetic layer proximate to a superconducting layer as in (Li1-xFexOH)FeS.  We propose that hydrogen bonding of the type N—H···Ch and O—H···Ch stabilize the growth of these layered iron chalcogenides. Due to the preparation from hydrothermal and solvothermal syntheses, the crystal growth of these layers involves several intermediate phases involving hydrogen bonding as evidenced by in situ X-ray diffraction studies. Finally, I will discuss some chemical bonding concepts that arise from group-subgroup relationships during phase transitions in these materials. It is clear that these layered chalcogenides support square lattices where electronic instabilities give way to either bonding distortions or superconductivity. 
11/11/2019 Prof. Sahar Sharifzadeh - Boston U.
First-Principles Studies of Excitonic Effects in Semiconductors
The ability to tune optical excitation (exciton) energies and direct their motion to a specific location will allow for unprecedented control over energy propagation and conversion in optoelectronic materials. In this talk, I will present our recent density functional theory and many-body perturbation theory studies aimed at understanding the nature and energetics of excitons within two classes of materials: organic and defective semiconductors. First, I will present our recent calculations aimed at understanding the spectroscopic properties of organic crystalline semiconductors, and tuning these properties for enhanced photovoltaic performance. By introducing a new analysis of the electron-hole correlation function, we demonstrate that excitons within organic crystals are delocalized with a degree of charge-transfer character, which can be controlled through solid-state morphology or change of conjugation length, suggesting a new strategy for the design of optoelectronic materials. Additionally, I will present investigations of the influence of point defects on the optoelectronic properties of bulk and monolayer semiconducting materials. For bulk GaN and monolayer GeSe, the predicted bandstructure and optical absorption spectrum indicate that introduction of the point defect can result in significant modification of the optoelectronic properties, particularly in 2D. A similar analysis of the electron-hole correlation function as above demonstrates how the Wannier exciton is perturbed by the presence of a defect.
Spring 2019
Date Speaker/Title/Abstract
01/28/2019 Reserved for Candidate Search
02/04/2019 Prof. Joanna Slusky - U. Kansas
The Structural Evolution of Outer Membrane Beta Barrels
Outer membrane proteins (OMPs) are the proteins in the surface of Gram-negative bacteria. These proteins have diverse functions but a single topology: the β-barrel. I will discuss the evolutionary pathways and origins of this topology. The mechanisms of diversification have implications for antibiotic resistance, repeat protein biogenesis, and the nucleation of outermembrane protein folding.   
02/11/2019 Reserved for Candidate Search
02/18/2019 Dr. Zac Ward - Oak Ridge National Laboratory
Synthesizing Discovery in Quantum Materials
The way that materials behave—are they magnetically complex, are they superconductors, are they structurally robust—can simply be thought of as a response to what the electrons in the material are doing. Controlling the atoms’ arrangement to one another in a crystal lattice changes where the electrons reside and how they interact with one another. If we can control the atomic structure and makeup, we can then manipulate electronic function. Exploiting this fact is particularly important in materials where strong electronic correlations are present. In these systems, the nearly degenerate energies of the spin, charge, and orbital order parameters mean that even slight variations to a single parameter can have a dramatic impact on what functional phenomena emerge. I will describe our recent development of new structural manipulation approaches that permit us access to never before possible lattice symmetries and atomic compositions in single crystal correlated oxides. You will see how these approaches are facilitating the rational design of orbital populations, spin anisotropy, crystal structure phase, and disorder-driven quantum critical behavior. We will close with a discussion of how this might impact our ability to probe fundamental physics problems in correlated materials and speculate on enabled functional applications. Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division
02/25/2019 Reserved for Candidate Search
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.

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