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 2021
Date Speaker/Title/Abstract
02/22/2021 Dr. Alan Robock, Rutgers University
Climatic and Humanitarian Impacts of Nuclear War
A nuclear war between any two nations, such as India and Pakistan, with each country using 50 Hiroshima-sized atom bombs as airbursts on urban areas, could inject 5 Tg of soot from the resulting fires into the stratosphere, so much smoke that the resulting climate change would be unprecedented in recorded human history.  Our climate model simulations find that the smoke would absorb sunlight, making it dark, cold, and dry at Earth’s surface and produce global-scale ozone depletion, with enhanced ultraviolet radiation reaching the surface.  The changes in temperature, precipitation, and sunlight from the climate model simulations, applied to crop models show that these perturbations would reduce global agricultural production of the major food crops for a decade. Since India and Pakistan now have more nuclear weapons with larger yields, and their cities are larger, even a war between them could produce emissions of 27 or even 47 Tg of soot. My current research project, being conducted jointly with scientists from the University of Colorado, Columbia University, and the National Center for Atmospheric Research, is examining in detail, with city firestorm and global climate models, various possible scenarios of nuclear war and their impacts on agriculture and the world food supply.  Using six crop models we have simulated the global impacts on the major cereals for the 5 Tg case.  The impact of the nuclear war simulated here, using much less than 1% of the global nuclear arsenal, could sentence a billion people now living marginal existences to starvation.  By year 5, maize and wheat availability would decrease by 13% globally and by more than 20% in 71 countries with a cumulative population of 1.3 billion people.  In view of increasing instability in South Asia, this study shows that a regional conflict using <1% of the worldwide nuclear arsenal could have adverse consequences for global food security unmatched in modern history. The greatest nuclear threat still comes from the United States and Russia.  Even the reduced arsenals that remain in 2020 due to the New START Treaty threaten the world with nuclear winter.  The world as we know it could end any day as a result of an accidental nuclear war between the United States and Russia.  With temperatures plunging below freezing, crops would die and massive starvation could kill most of humanity. As a result of international negotiations pushed by civil society led by the International Campaign to Abolish Nuclear Weapons (ICAN), and referencing our work, the United Nations passed a Treaty to Ban Nuclear Weapons on July 7, 2017.  On December 10, 2017, ICAN accepted the Nobel Peace Prize “for its work to draw attention to the catastrophic humanitarian consequences of any use of nuclear weapons and for its ground-breaking efforts to achieve a treaty-based prohibition of such weapons.”  Will humanity now pressure the United States and the other eight nuclear nations to sign this treaty?  The Physicists Coalition for Nuclear Threat Reduction is working to make that happen. Bio: Dr. Alan Robock is a distinguished professor of climate science in the Department of Environmental Sciences at Rutgers University, associate editor of the journal Reviews of Geophysics, Lead Author of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, and a former Peace Corps Volunteer in the Philippines, and has been researching the climatic and agricultural impacts of nuclear war for the past 35 years. Join Zoom Meeting: https://umsystem.zoom.us/j/94685134794?pwd=c0x4bzlmNjlrck10Wmp6SWp2YzkvUT09
03/29/2021 John F. Marko, Northwestern University
Loop extrusion, chromatin crosslinking, and the geometry, topology and mechanics of chromosomes and nuclei
The chromosomes of eukaryotic cells are based on tremendously long DNA molecules that must be replicated and then physically separated to allow successful cell division.  I will discuss what we have learned about chromosome structure from our group's biophysical experiments and mathematical modeling of chromosome structure. A key emerging feature of chromosome organization is the role of active chromatin loop formation, or "loop extrusion" as a mechanism leading to chromosome compaction, individualization, and segregation.  I will discuss a number of aspects of the SMC complexes thought to be the loop-extruding elements. I will also discuss our group's studies of the role of chromosomal epigenetic marks in control of the structure and integrity of the cell nucleus.
04/05/2021 Prof. Dam Thanh Son, University of Chicago
04/12/2021 Smitha Vishveshwara, UIUC
Exploring anyons and black holes-like dynamics in flatland
Fall 2020
Date Speaker/Title/Abstract
10/12/2020 Prof. Joel Helton, United States Naval Academy, Annapolis, MD
Frustrated magnetism in Co3V2O8: competing interactions on a S=3/2 kagome staircase lattice
Cobalt vanadate, Co3V2O8 features S=3/2 magnetic moments on a three-dimensional kagome staircase lattice. Nearest-neighbor magnetic interactions are weak, leading to a manifold of nearly degenerate ordered spin density wave states driven by competition between ferromagnetic and antiferromagnetic interactions. Subtle changes in temperature and magnetic field drive transitions between these ordered states. Spin wave measurements also reveal a particularly strong anisotropic Dzyaloshinskii-Moriya term in the Hamiltonian. In addition to serving as a playground for exploring classical frustration due to competing interactions, deducing the full spin Hamiltonian of Co3V2O8 might offer new insights into the nearly isostructural multiferroic  Ni3V2O8. Please email umcasphysics@missouri.edu for zoom link for event.
10/19/2020 Prof. Haojing Yan, MU Physics
Nobel Prize in Physics 2020: Black Holes
The Nobel Prize in Physics 2020 was awarded to three scientists for their contributions to our understanding black  holes, one of the most amazing phenomena in the universe. Half of the prize went to Roger Penrose for the discovery that black hole formation is a robust prediction of the general theory of relativity". The other half was awarded jointly to Reinhard Genzel and Andrea Ghez "for the discovery of a supermassive compact object at the center of our galaxy." The "compact object" here is a black hole of 4.1 million times of solar mass. This talk is to present and to explain their research, and to point out a mystery for more than five decades that now might be on the verge of being solved. Seminar is open to all, please email umcasphysics@missouri.edu for zoom link.
Spring 2020
Date Speaker/Title/Abstract
01/27/2020 Prof. Mark Siemens - U. Denver
Quantum Turbulent Structure in Light
A random superposition of plane waves is known to be threaded with vortex line singularities which form complicated tangles and obey strict topological rules. In this work, we use both numerical simulations of random waves and experiments on laser speckle to observe and characterize the dynamics of the vortex tangles. We find that the velocity statistics of the vortices in random waves match those of turbulent quantum fluids such as superfluid helium and atomic Bose-Einstein condensates (Physical Review Letters 122, 044301 2019). These statistics are shown to be independent of system scale. These results raise deep questions about the role of nonlinearity in the structure of turbulence and the general nature of quantum chaos. 
02/10/2020 Prof. Bharat Ratra - Kansas State U.
The Accelerating Expanding Universe: Dark Matter, Dark Energy, and Einstein’s Cosmological Constant, or Why Jim Peebles was Awarded Half of the 2019 Physics Nobel Prize.
Dark energy is the leading candidate for the mechanism that is responsible for causing the cosmological expansion to accelerate. In this non-technical talk, Bharat Ratra will describe the astronomical data which persuade cosmologists that (as yet not directly detected) dark energy and dark matter are by far the main components of the energy budget of the universe at the present time. He will review how these observations have led to the development of a quantitative "standard" model of cosmology that describes the evolution of the universe from an early epoch of inflation to the complex hierarchy of structure seen today. He will also discuss the basic physics, and the history of ideas (many developed by Jim Peebles), on which this model is based.
03/09/2020 Prof. Bhupal Dev - Washington Univ. St Louis
Ghost Hunting: The Story of Neutrinos and the Cosmos
Neutrinos are one of the most abundant particles in the Universe and are constantly streaming through our bodies unnoticed. We will discuss how these teeny-tiny particles could help us unravel some well-kept secrets of the cosmos. Because of their extreme shyness to interact with ordinary matter, it takes extraordinary ingenuity and humongous detectors to catch these ghostlike particles in earthly experiments. This was a rewarding endeavor and led to the discovery of neutrino oscillations -- the first-ever laboratory evidence for physics beyond the Standard Model. However, the neutrino game is far from being over; rather, this is just the beginning of a remarkable journey into the enchanting world of neutrinos, with many current and upcoming experiments poised to make fundamental discoveries. In particular, we will highlight some recent exciting news from Antarctica that has ushered a new era in neutrino astrophysics and could revolutionize our understanding of the Universe.
04/06/2020 Prof. Michael Geller - U. Georgia
When Will Quantum Computers be Useful?
Several companies have recently developed small and intermediate-scale quantum computing devices. These systems lack the size and accuracy required for error correction, which would seem to be necessary for practical applications. However there is now a tremendous hope--and hype--for the possibility of finding near-term applications that do not require the overhead required for full error correction. In this talk I will review the current state of the field and discuss these recent developments. I will also discuss our own approach, called the single-excitation subspace method, to this challenge. 
04/20/2020 Prof. Maxim Lyutikov - Purdue U.
The double pulsar: tomography of pulsar magnetosphere and a new test of General Relativity
The long awaited discovery of the binary radio pulsar system, PSR J0737-3039A/B, surpassed most expectations, both theoretical and observational, as a tool to probe general relativity, stellar evolution and pulsar theories. Unexpectedly, the faster pulsar A is eclipsed once per orbit while the slower pulsar B shows orbital-dependent variations of intensity. I will describe a model of eclipses which reproduces the complicated observed light curve down to intricate details. This proves the long standing assumption of dipolar magnetic fields of neutron stars and gives a tool to probe details of magnetospheric structure and pulsar emission generation mechanisms. The model also provides a quantitative measurement of relativistic spin precession and offers a new test of theories of gravity.
04/27/2020 Prof. Aurora Pribram-Jones - U. California, Merced
Weak, Physical, and strong Interaction via the Generalized Thermal Adiabatic Connection
05/04/2020 Prof. Caitlin Casey - UT Austin
Dust Obscured Galaxies
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. 


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