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 2018
Date Speaker/Title/Abstract
04/16/2018 Prof. Daniel Wang - U. Massachusetts-Amherst
Probing Astrophysical Black Holes via Gravitational Lensing
Black holes (BHs) represent the most extreme objects in the universe and play an important role in astrophysics. We have been exploring various ideas of using gravitational lensing to probe the population and astrophysics of BHs. Supermassive BHs of million solar masses or greater are usually detected as active galactic nuclei (AGN). We show that the innermost X-ray-emitting structure of AGN can be greatly amplified and hence effectively probed by microlensing of nearby foreground stars. For stellar mass BHs in our Galaxy,  we may estimate their overall population via their astrometric microlensing effect on background sources. This capability is within the reach of available near-IR/radio interferometry facilities. Particularly interesting is the possibility to detect a concentration of stellar mass BHs (including ones of ~30 solar masses, similar to those discovered recently via gravitational waves) around the very center of our Galaxy. Furthermore, we can effectively study the formation and evolution of both stellar mass and supermassive BHs at high redshifts via strong gravitational lensing by foreground massive galaxies or galaxy clusters.
04/23/2018 Dr. Charles Majkrzak - NIST
What can the reflection of neutrons reveal about the structure of hard and soft condensed matter?
In general, direct imaging methods such as optical, atomic force, and electron microscopy are powerful and of wide-ranging application for obtaining information about the shapes of materials on micron, nanometer, and atomic length scales, respectively. Nonetheless, as is well known, there are limitations to what can be discerned by such techniques and complementary scattering measurements are often necessary to obtain a more detailed compositional picture. In particular, for studies of materials possessing multi-layered lamellar morphologies, diffraction measurements with x-rays and neutrons at glancing angles of incidence can yield unique quantitative structural information on a nanometer length scale. Diffraction at relatively low values of wavevector transfer Q -- where the scattering potential of a material object can be treated as being continuous and associated with a corresponding refractive index (as in ordinary light optics) -- has come to be referred to specifically as reflectometry. From measurements of the reflectivity as a function of Q in the specular condition (in which the angles of incidence and reflection are of equal magnitude), the depth profile of the scattering potential density along the nominal surface normal can be reconstructed and subsequently related to the actual material composition distribution. Neutron reflectivity studies of a number of hard and soft condensed matter systems of current interest -- including examples in magnetism, polymer science, and biology -- will be examined. Possible future advances in neutron reflectometry as a probe to investigate the structure of materials will also be discussed. The presentation will be directed toward a broader audience of scientists, including students, and not intended primarily for scattering experts.
Fall 2017
Date Speaker/Title/Abstract
08/22/2017 Prof. George Zweig - MIT
Special Colloquium: Finding the Equations of the Organ of Corti
Measurements of basilar membrane motion are used to derive a nonlinear model of three dimensional cochlear mechanics.  The talk is aimed at Physics graduate students who have an interest in biology.  Biologists with sufficient mathematical skills may also find the talk of interest. Venue: Physics Library 223 
08/28/2017 Prof. Tabbetha Dobbins - Rowan Univ.
New Possibilities for Understanding Complex Metal Hydrides via Synchrotron X-ray and Neutron Scattering Studies
This research seeks to understand the role of catalysts and nanostructuring plays in structure and dynamics in hydrides. The talk will include results of X-ray absorption spectroscopy (XAS), Ultrasmall-angle X-ray scattering (USAXS) and Quasi-elastic neutron scattering (QENS) studies.  XAS is used to examine the chemical interaction of catalysts (TiCl3) with the host hydride powder (NaAlH4). USAXS is used to examine the morphological changes in the hydrides both with and without catalysts.  Finally, QENS provides information on hydrogen dynamics as a result of catalyst addition and also due to nanostructuring.  Our research uses several specialized synchrotron X-ray and neutron techniques to elucidate the interactions of catalysts with the host hydride structures.  This presentation will describe those techniques and their use in describing the physical and chemical processes occurring in the materials.
09/04/2017 No Colloquium - Labor Day
09/11/2017 Please note: there is a special colloquium on 9/12
09/12/2017 Dr. Alan Van Nevel, NAVAIR Weapons Division
Special O. M. Stewart Colloquium: Research at the Naval Air Warfare Center, Weapons Division
Special O. M. Stewart Colloquium; Venue: Physics Library, Rm 223 A In this discussion, Dr. Alan Van Nevel will present a brief overview of the research directorate at the Naval Air Warfare Center, Weapons Division at China Lake, CA and highlight ongoing efforts within the directorate.   Topics to be covered include autonomous systems with collaborative control,  atmospheric characterization, Pulsed Electron Deposition of Transparent Conductive Coatings, quantum cascade laser modelling, and spintronics and spin valve research. The goal is to highlight areas of active research and interest at NAWCWD, to foster potential interactions and collaboration between NAWCWD researchers and Mizzou faculty.
09/25/2017 Prof. Haojing Yan -MU
Study of Galaxy Formation in the New Era
10/09/2017 Prof. Zhigang Chen - San Francisco U.
Novel phenomena in photonic graphene
Photonic graphene, the photonic analog of graphene constructed with waveguide arrays arranged in honeycomb lattice (HCL) geometry, has provided a powerful platform to emulate graphene physics while discovering new phenomena that would otherwise be inaccessible in real graphene. In recent studies, artificial HCLs have been successfully employed to investigate a variety of fundamental phenomena such as strong sublattice symmetry breaking, strain-induced pseudomagnetic fields, Berry curvature effects and photonic topological insulators. In this talk, I will discuss some of our recent work based on photonic graphene, including unconventional edge states, the pseudospin-mediated vortex generation and topological charge flipping, the Aharonov-Bohm-like interferences, and valley Bloch oscillations and Landau-Zener tunneling in photonic graphene that illustrates the important influence of the lattice geometry and wavepacket dynamics at Dirac points.  Venue: 223 A Physics Building
10/16/2017 Prof. Andrew Gu - MU
Nanopore force spectroscopy of biomolecular interactions and application in biomedical detection
Precision medicine is a new disease (in particular cancer) treatment and prevention approach via personalized strategy based on the molecular abnormalities in genes, proteins, and metabolites for each person. President Obama’s $215 million Precision Medicine Initiative® (2015) is motivating the development of new-generation technologies for accurate measurement and systematic analysis of individual’s genetic, epigenetic, proteomic and metabolic profiles. Our lab is working on a nanopore-based single-molecule technology (electrochemical measurement) for rapid, label-free and low-cost DNA sequencing and various genetic, epigenetic, and proteomic detections. The new generation of programmable nanopore biosensors is being combined with smart polymers and microfludics to create robust chip device for medical diagnosis, treatment, and high-throughput screening at the molecular level. These explorations are exemplified by, but not limited to, 1) the nanopore detection of cancer-associated circulating non-coding RNAs epigenetic biomarkers such as microRNAs, offering a non-invasive, cost-effective and accurate tools for confirmatory cancer detection; and 2) manopore force toolbox: from mechanistic study of RNA folding/unfolding to clinical detection of RNA biomarkers. The overall research has broad impact toward precision medicine and life science.  Venure: Physics Library (223 A)
10/23/2017 Dr. Samindranath Mitra, Editor PRL
Physics after the lab and the desk: Your work in PRL
Physics research takes place mostly at your desk, at the keyboard, in the lab. You communicate results through posters, talks, and papers -- leading to, hopefully, wide dissemination and recognition. The sequence entails interacting with journal editors, referees, conference chairs, journalists, and so on. I will focus on this post-research collaborative process in physics, primarily through the lens that is Physical Review Letters.
10/30/2017 Profs. Mirela and Lorin Milescu - MU
COLLOQUIUM CANCELLED
11/06/2017 Prof. Gerardo Ortiz - Indiana U.
The Majorana mysteries
On March 25th 1938 at the age of 32 years Ettore Majorana vanished, under mysterious circumstances, without leaving a trace. Only recently his name re-emerged as the Majorana fermion, a quasi-particle excitation that represents it own anti-quasi-particle, has been claimed to be detected by several experimental groups. Majorana fermions dance in a superfluid background. They have the potential to make quantum computers robust because of the special topological property of non-Abelian braiding, generated whenever Majorana fermions are transported around each other. But, what property distinguishes topologically trivial from non-trivial superfluids, supposedly hosting Majorana fermions? What is the meaning and fate of such excitations in closed, number conserving, interacting fermionic superfluids? I will attempt to answer these questions from both basic physics principles and concrete model perspectives. In particular, I will discuss a novel route to topological superfluidity realizable in repulsive ultracold alkaline-earth atomic systems, and will propose several novel experimental probes to unveil the topological superfluid state.
12/04/2017 Matthew Maschmann - U. Missouri
Recent Advances in Carbon Nanotube Forests
The promise of carbon nanotubes (CNTs) as a “wonder material” has thus far failed to live up to the initial hype. Individual CNTs demonstrate high modulus (1TPa) and strength (50 GPa) while maintaining flexibility. Thermal and electrical transport properties likewise exceed conventional engineering materials. When dense populations of CNTs are synthesized into vertically-oriented films for large scale applications, however, the beneficial properties fail to scale in a in a volumetric manner. These CNT populations, known as CNT forests or CNT arrays, feature a complex and intertwined morphology which tend to dictate their ensemble properties. In this talk, I will discuss mechanisms responsible for inhomogeneous CNT forest morphology and highlight simulation, synthesis, and processing advances from within my research group towards the goal of optimizing CNT forest properties for specific applications.
Spring 2017
Date Speaker/Title/Abstract
01/26/2017 Dr. Yicheng Guo - UC Santa Cruz
Special O. M. Stewart Colloquium: Internal Structures of Distant Galaxies: What Giant Star-forming Clumps Tell Us About the Assembly Histories of Galaxies
Venue: Physics Library Studies of galaxy formation and evolution have made remarkable progress in the past two decades. Thanks to modern telescopes, such as HST, Keck, Spitzer, and SDSS, the integrated properties of distant galaxies -- mass, size, color, and shape -- have been accurately measured and used to display a broad picture of galaxy assembly across cosmic time. With the powers of current telescopes and super computers, extragalactic astronomy has come to a critical point to study the internal structures of distant galaxies. In this talk, I will present our work of studying a common and important internal feature of distant star-forming galaxies, namely giant star-forming clumps, and show how the internal structure significantly improves our understanding of the accretion history of galaxies, evolution of disks, and formation of bulges. I use the data from CANDELS, the largest HST survey program, and state-of-the-art cosmological hydrodynamic simulations to answer three questions: (1) When and how were clumps formed? (2) How did they evolve once being formed? (3) What do clumps tell us about the physical mechanisms that regulate star formation? The three aspects reveal clues of how galaxies evolve from small, irregular blobs in early universe into today's Hubble Sequence. I will also discuss how future multi-wavelength facilities, e.g., JWST, WFIRST, and SKA, would provide new, key information to understand galaxies' internal structures.
01/30/2017 Prof. Martin Ulmschneider - Johns Hopkins U.
Validating Atomic Detail Peptide Partitioning Simulations Using Synchrotron Radiation Circular Dichroism Spectroscopy
Studying how flexible membrane-active peptides interact with fluid-phase biological membranes is important for unraveling the mechanisms of antimicrobial and cell-penetrating peptide activity, as well as understanding how proteins fold and assemble in lipid bilayers. However, the fluidity, chemical complexity, and nanometer-scale dimensions of the membrane environment make studies of peptide-membrane interactions a challenging task. Here we show that the insertion pathway, transfer energetics, and partitioning kinetics of membrane active peptides into lipid bilayers can be obtained by a combination of synchrotron radiation circular dichroism spectroscopy and direct equilibrium atomic resolution molecular dynamics simulations [1,2]. Remarkably, the results are in close quantitative agreement with in vitro translocon experiments. The insertion probability as a function of peptide length follows two-state Boltzmann statistics and reveal many hitherto unknown atomic-resolution details about the partitioning process. The approach presented provides a useful tool for studying of water-to-bilayer transfer properties of membrane active peptides. 1.  J.P. Ulmschneider, J.C. Smith, S.H. White & M.B. Ulmschneider. Journal of the American Chemical Society 2011133, pp15487–15495 2. M.B. Ulmschneider, J.P. Ulmschneider, N. Schiller, B.A. Wallace, G. von Heijne & S.H. White. Nature Communications 2014, 5 (4863), pp1-10
02/06/2017 Vivian U - UC Riverside
Gas Flows: from Black Holes to Galaxies
The tight correlations between black hole mass and the bulge properties of its host galaxy suggest that black holes and their hosts coevolve.  However, the drastically different size and mass scales of these two structures make a physical explanation of the correlation challenging. Gas plays a prominent role in galaxy evolution by fueling supermassive black holes and star formation; feedback processes quench activity by heating or removing the gas. Tracing nuclear gas kinematics at small scales presents the most direct way to examine the feeding and feedback associated with massive black holes, particularly in the luminous regime where mass accretion is happening most rapidly. In this talk, I will present our ongoing efforts to explore the broad line region structure and dynamics in luminous Seyfert galaxies using reverberation mapping in a new campaign of the Lick AGN Monitoring Project. I will also discuss results from the Keck OSIRIS AO LIRGs Analysis (KOALA) Survey that highlight the properties of molecular outflows driven by AGN and starbursts at the heart of nearby luminous infrared galaxies. I will demonstrate the power of near-infrared diagnostics of shocks associated with nuclear winds that will become indispensable for understanding the interstellar medium as we enter an exciting era of astronomy with the imminence of JWST, 30-meter class telescopes, and beyond.
02/13/2017 Patrick Kelly - UC Berkeley
Using Galaxy Cluster Lenses as Extreme Probes
Distant galaxies can be highly magnified by foreground galaxy clusters, making cluster lenses powerful tools for studying the high-redshift universe. The James Webb Space Telescope, when pointed toward foreground clusters of galaxies, will be sensitive to even low-luminosity galaxies at redshifts z > 6 (~35th magnitude) thought to drive reionization.  In regions of high amplification, however, cluster magnification maps, derived from models of cluster dark matter distributions, show strong disagreements. I will describe the first strongly lensed, multiply imaged supernova (SN), which appeared in the MACS1149 cluster field in late 2014 and then again, offset by ~8 arcseconds, in late 2015.  The time delay between the appearances of the SN disagrees with most but not all predictions, and illustrates a promising method for identifying the best assumptions about the distribution of dark matter in clusters.  I will next discuss a new and direct way to study individual massive stars near cosmic “high noon” (z~2), as well as the composition of dark matter in the form of compact objects.  Detections of hundreds of thousands of SNe and thousands of lensed transients by the Large Synoptic Survey Telescope and the Wide-Field Infrared Survey Telescope, with follow up from X-ray to radio wavelengths, will allow new insights into star formation and stellar evolution beginning at z~15-20, as well as the nature of dark matter and dark energy.
02/16/2017 Daniel Jacobs - Arizona State Univ.
Special O. M. Stewart Colloquium - HERA: the Next Generation 21cm Reionization Array
Venue: Room 126 Physics Building The first 500 million years after the big bang is one of the last unexplored periods of cosmic history. During that time Hydrogen combined from the primordial plasma and condensed into the first stars and black holes which soon ionized the remaining hydrogen.  This rich history is accessible via the hydrogen 21cm line which can theoretically trace out structure in three dimensions before, during, and after reionization.  Observations by new low frequency radio telescopes like the Hydrogen Epoch of Reionization Array will provide a unique view of the first stars and galaxies complementary to deep observations of the most distant galaxies; there is also the opportunity to tighten constraints on cosmological parameters. HERA is a second generation 21cm reionization telescope currently under construction in South Africa.   The experimental challenge is in discriminating faint background from bright foreground; to meet this challenge we are developing new methods for precision instrument calibration and analysis. In this talk we’ll learn about some of the early 21cm results, the HERA science goals, some instrumental and analysis challenges, and look briefly to the future.
02/20/2017 Reserved - Astro Search
No Colloquium

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