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).
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.
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.
Prof. Michael Geller - U. Georgia
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.
Prof. Aurora Pribram-Jones - U. California, Merced
Weak, Physical, and strong Interaction via the Generalized Thermal Adiabatic Connection
Prof. Caitlin Casey - UT Austin
Dust Obscured Galaxies
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.
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.
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.
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.
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.
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.
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.
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.
|01/28/2019||Reserved for Candidate Search|
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|
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|
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.