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Events: Departmental Colloquia

  • SUSY Rilles: A Speculation on Superstring Signatures in the Cosmic Microwave Background

    Guest: Prof. James Gates, Jr., Department of Physics, Brown University
    Thursday, September 3, 2020 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    We speculate about the imprint of higher spin supermultiplets on cosmological correlators, i.e., the non- Gaussianity of the cosmic microwave background. Supersymmetry is used as a guide to introduce the contribution of fermionic higher spin particles, which have been neglected thus far in the literature. We compute the curvature perturbation 3-point function for supersymmetric higher-spin particle exchanges and find the known P(cos) angular dependence is accompanied by superpartner contributions.

    Thursday, September 3, at 3:55 PM. Join Zoom Meeting: https://zoom.us/j/99808158494

     

  • Uncovering a New Universality at a 1st Order Phase Transition

    Guest: Prof. David Landau, Center for Simulational Physics, University of Georgia
    Thursday, August 27, 2020 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Understanding principles of critical exponents and Universality at 2nd order phase transitions was a triumph of late 20th century physics. In contrast, 1st order transitions appeared to be boring. We will show how Monte Carlo simulations, together with experiment and theory, reveal unexpected behavior at a 1st order phase transition. To do this we examine the 1st-order “spin-flop” transition between the Ising-like antiferromagnetic state and the canted, XY-like state found in many anisotropic antiferromagnetic materials. Finite-size scaling for a 1st-order phase transition where a continuous symmetry is broken is developed using an approximation of Gaussian probability distributions. Predictions are compared with data from Monte Carlo simulations of an anisotropic Heisenberg antiferromagnet in a magnetic field. Our theory predicts that for large linear dimension the field dependence of all moments of the order parameters as well as the 4th-order cumulants exhibit universal intersections that can be expressed in terms of a factor q that characterizes the relative degeneracy of the ordered phases. Our theory yields simply q = π, independent of temperature!, and the agreement with numerical data implies a heretofore unknown Universality for 1st-order phase transitions.
     
    Thursday, August 27, at 3:55PM.
  • GPU Molecular Dynamics: Algorithims, Performance and Examples

    Guest: Prof. Dennis C. Rapaport, Department of Physics, Bar-llan University, Israel
    Thursday, February 20, 2020 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Computer studies of complex physical phenomena, especially those utilizing MD (molecular dynamics) simulation, are often resource intensive. Supercomputer architecture, now mainly based on highly parallel GPUs (graphics processing units), is becoming increasingly complicated, so that designing efficient algorithms is a far more difficult task than it once was. Following a brief historical introduction, GPU architecture will be outlined, and novel GPU algorithms surveyed. MD methods will then be reviewed, and some of the unique demands they impose on the GPU described and resolved, with emphasis on the performance gains. The talk concludes with a discussion of several MD studies of emergent phenomena. Lessons learned while adapting MD for the GPU should be valuable in other contexts where compatibility between algorithms and hardware may not be apparent.

  • Non-Abelian States Beyond Localized Majorana Fermions

    Guest: Prof. Luiz Santos, Department of Physics, Emory University
    Thursday, February 13, 2020 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Superconductivity and topological order are two of the fundamental pillars that support our understanding of phases of matter. The intersection of these two areas, topological superconductivity, is a fertile ground for scientific discoveries. In particular, topological superconductors have the important property of hosting non-Abelian states, the simplest example of which are Majorana fermion zero modes localized on the core of superconducting vortices. Non-Abelian states have attracted much attention as potential building blocks for fault-tolerant quantum computation and, consequently, much intellectual activity has been focused on understanding novel mechanisms to realize non-Abelian states. This colloquium will discuss two scenarios for the realization of non-Abelian quasiparticles beyond localized Majorana zero modes. First, I will describe how the interplay of topological order and superconductivity can lead to the realization of parafermions on interfaces and heterostructures formed by fractional quantum Hall states and superconductors. Parafermions realize a protected ground state degeneracy, in which rotations of the degenerate quantum states are realized by swapping pairs of parafermions. Second, I will introduce the concept of a pair-density wave in even denominator paired quantum Hall systems as a state where the pairing order parameter breaks rotation symmetry. Remarkably, in this setting, the pairing function has domain walls that give rise to a Fermi sea of delocalized Majorana fermions, which is an example of a symmetry protected gapless state. Along the way, I will discuss connections of this pair-density wave theory with recent experiments in nematic fractional quantum Hall states.

     

    1. Phys. Rev. X 9, 021047 (2019)
    2. Phys. Rev. Lett. 118, 136801 (2017) 
    3. arXiv: 1906.07188 to appear in Phys. Rev. Research
  • Laboratory Astrophysics Studies along the Cosmic Cycle of Gas (Part I)

    Guest: Dr. Daniel Wolf Savin, Columbia Astrophysics Laboratory, New York, NY
    Thursday, February 6, 2020 2:30 pm - 3:30 pm
    Location: Physics Auditorium (202)

    Tracing the evolution of baryonic matter from atoms in space to stars and planets hinges on an accurate understanding of the underlying physics controlling the properties of the gas at every step along this pathway. Here I will explain some of the key epochs in this cosmic cycle and highlight our laboratory studies into the underlying atomic, molecular, plasma, and surface physics which control the observed properties of the cosmos.

  • Engineering Oxide Thin Films at the Atomic Level for New Electronic and Energy Applications

    Guest: Prof. Ryan Comes, Auburn University, Department of Physics
    Thursday, January 30, 2020 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Complex oxides comprised of multiple positively charged metal cations exhibit a host of intriguing and useful properties for new technologies. Perovskite oxides with the chemical formula ABO3 and spinel oxides with the formula AB2O4 have some of the richest behavior. These materials may be metallic, semiconducting, or insulating, and exhibit ferroelectricity, with a built-in electric polarization, ferromagnetism, or superconductivity. This combination of properties in a single class of materials offers rich opportunities for engineering of unusual combinations of behavior through the design of multi-layer thin film materials. Through the use of molecular beam epitaxy (MBE), we are able to engineer these materials down to the atomic level so that interfaces between two different materials can be controlled to produce desirable properties. In this talk I will present two examples of this type of interfacial engineering, showing how we can design, model, and characterize these properties through a wide variety of techniques. I will first discuss our work on spinel and perovskite oxide nanocomposites that can be used in the oxygen reduction and oxygen evolution reactions. Using a combination of x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and spectroscopic ellipsometry we have answered fundamental questions about the properties of CoMn2O4 and MnFe2O4. Ongoing work focuses on integrating these materials with perovskites such as LaNiO3 and LaFeO3 to produce bifunctional catalysts. Our second project focuses on the synthesis of defect-free SrTiO3 and Sr(Ti,Nb)O3 thin films using hybrid MBE. Using XPS surface studies, we have answered fundamental questions regarding this emerging growth technique. Ongoing work focuses on the use of these materials to produce novel oxide heterostructures for topological phases, and spintronic devices.

  • Spintronics with 2D Materials: New Functionalities with Heterostructures

    Guest: Dr. Yunqiu (Kelly) Luo, Cornell University
    Thursday, January 23, 2020 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    In this talk, I’ll first briefly review graphene spintronics and the ‘hallmark’ Hanle spin precession measurement in non-local spin valves. Then I’ll discuss our recent works in hybrid 2D optospintronic systems, such as the first demonstration of opto-valleytronic spin injection across a monolayer transition-metal dichalcogenide (TMD)/graphene interface. We confirmed the full process of optical spin injection, lateral spin transport and electrical spin detection up to room temperature using Hanle spin precession and showed such process can be controlled by photon helicity and photon energy. Next, I will discuss our current work investigating the origin of such charge and spin transfer processes across TMD/graphene interfaces. We observed an intriguing photoconductance bias and gate dependence in a dual-gated MoS2/ graphene field-effect device, indicating the dominance of a vdW photothermoelectric effect. In addition, we study the ultrafast photon-spin transfer process across a monolayer WSe2/ graphene interface using time resolved Kerr rotation and photocurrent microscopy. We discovered a highly efficient spin and charge transfer dynamics from WSe2 to graphene that last less than 10 ps. At last, if time permits, I’ll briefly discuss our recent transport work that unveils a strong and highly tunable spin-lifetime anisotropy in dual-gated bilayer graphene.

  • Strongly Correlated Oxides for Artificial Intelligence

    Guest: Prof. Shriram Ramanathan, Materials Engineering, Purdue University
    Thursday, January 16, 2020 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    AI machines and neuromorphic computers are rapidly growing research directions in the physical sciences disciplines. Correlated electron systems could potentially serve as building blocks for AI hardware due to their highly tunable electronic band structures and rapid environmental response. Using high- pressure synthesized perovskite nickelates as a model system, we will first discuss insulator-metal transitions that are controlled by hydrogen doping. From understanding the binding of the charge carriers to the lattice, we will describe experiments in nickelates that demonstrate neuromorphic learning. We will conclude with some examples of how basic research on the electronic structure of correlated oxides naturally pave the way for their use in artificial neural networks and reconfigurable photonic devices.

  • Playing with Photons in Flatland: Controlling Light and Matter in Two-Dimensional Materials

    Guest: Prof. Nathaniel P Stern, Department of Physics and Astronomy, Northwestern University
    Thursday, November 21, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Light is a powerful tool of science. The quantum conception of light consisting of particles of discrete energy, or photons, underlies its interaction with matter. For solid materials, this understanding has led to transformational applications both as conventional as sensor and display technologies and as extraordinary as lasers. Despite this ubiquity, advances in materials science continue to reveal nuances in the interaction of light with matter. The emergence of layered two-dimensional materials of atomic-scale thickness presents a new two-dimensional landscape in which to play with the interaction between light and matter. These nanomaterials at the extreme limit of surface-to-volume ratio exhibit rich optical phenomenology such as layer dependent bandgaps and degenerate, but distinct, optically excited states. The unique features of atomically-thin materials suggest that these layered systems can be exploited to achieve new regimes of light-matter interactions. In this presentation, I will discuss photonic dressed states in monolayer semiconductors in which excitations of matter become entwined with the photon field. In particular, I will describe the emergence of spin- polarized half-light, half matter quasiparticles, or exciton-polaritons in transition metal dichalcogenides embedded in photonic microcavities. I will trace these novel photonic dressed states across strong and weak regimes, revealing quantum particles and quantum manipulation and adding to the toolbox for engineering novel applications harnessing the unique properties of low-dimensional nanomaterials.

  • Comets and the origins of water in our solar system

    Guest: Prof. Dennis Bodewits, Department of Physics, Auburn University
    Thursday, November 14, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Comets are considered primitive left-overs from the era planet formation. Most comet science questions therefore revolve around whether observed properties are primordial, i.e. representative of conditions during the era of planet formation, or whether they are caused by subsequent processing. Comets may also have delivered water and complex molecules to Earth and other planets in our solar system. Finally, the discovery that our solar system is frequently visited by interstellar comets places comet science at the forefront of astrobiology. This talk will take attendees on a tour of what we know about comets, what mysteries we need to solve, and how future spacecraft and telescopes could help us answer our questions.

    Rosetta-mission-700x467.jpg
    The Rosetta mission at comet 67P/Churyumov-Gerasimenko
  • Squeezing the most of Heisenberg using a Bose-Einstein condensate

    Guest: Prof Michael Chapman, School of Physics, Georgia Institute of Technology
    Thursday, November 7, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Heisenberg’s uncertainty principle establishes a “standard quantum limit” of measurement precision that sets a bound on many precision measurements including atomic clocks and gravitational wave detectors. Using a special class of entangled quantum states known as squeezed states, it is possible to exceed the standard quantum limit. I will discuss our experiments investigating spin-1 atomic Bose-Einstein condensates in which non-equilibrium dynamical evolution creates spin-squeezed states with uncertainties an order of magnitude below the standard quantum limit. Additionally, we have developed novel quantum control techniques for the spin states and investigated Kibble-Zurek universality by quenching the spin system across a quantum phase transition. These experiments demonstrate new methods of manipulating out-of-equilibrium quantum systems, drawing together ideas from classical Hamiltonian dynamics and quantum squeezing of collective states.

  • Matter Interactions at the Nanoscale and Beyond

    Guest: Prof. Li Yang, Department of Physics, Institute of Materials Science and Engineering, Washington University
    Thursday, October 31, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    In this talk, I will start from a general picture of light-matter interactions, such as quasiparticles and excitons, in solids and how to calculate them by first-principles approaches. Then I will focus on light-matter interactions of nanoscale materials, in which the reduced dimensionality substantially enhances many-electron interactions by orders of magnitude and results in unique excited-state properties, such as strongly polarized excitons and exciton liquids. By clarifying and calculating electron-electron, electron-hole, and electron-plasmon interactions, we can accurately explain many important measurements and provide new ideas to engineer light-matter interactions for exploring new science and realizing device and energy applications. Finally, beyond light-matter interactions, I will show how to combine different levels of first-principles tools and models to predict a wide range of electric and magnetic polarizations of solids and their applications.

  • Theory and Simulations of Heat Conduction in Anharmonic Solids

    Guest: Prof. Jianjun Dong, Department of Physics, Auburn University
    Thursday, October 24, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    At the dawn of quantum physics, RE Peierls proposed to combine the concept of quanta of lattice vibration, i.e. phonons, with the particle Boltzmann Transport Equation (BTE) theory to describe thermal transport processes at the microscopic level. Since then, the phonon gas (PG) model has served as the theoretical foundation to calculate and interpret thermal transport properties of solids. In this talk, I will present some of our recent work of implantation and further development of atom-scale computational methodologies for lattice thermal conductivity calculations. First, I will discuss the strengths and weaknesses of three commonly adopted computational methods, namely, (1) the non-equilibrium molecular dynamics simulation method, (2) the equilibrium fluctuation-dissipation theory, such as Green-Kubo formalism, and (3) the phonon BTE theory. Then, I will highlight some of our newly discovered heat transfer mechanisms, including the phonon-polariton effects on bulk lattice thermal conductivity at high temperatures, and the coupling-decoupling mechanism for interfacial heat transfer cross weakly bonded molecular interfaces. Finally, I will discuss the breakdown conditions of the PG model based on our newly proposed vibration Fokker-Planck Equation (FPE) theory, and the implications to calculations of thermal conductivity at high temperatures and/or of solids with strongly localized vibrational modes.

    References:
    Yi Zeng, Jianjun Dong, and Jay M. Khodadadi, “Theory and simulations of thermal conductance of flexible molecular interfaces”, under review.
    Yi Zeng and Jianjun Dong, “Fokker-Planck equation for lattice vibration: Stochastic dynamics and thermal conductivity", Phys. Rev. B 99, 014306 (2019).
    A.M. Hofmeister, Jianjun Dong, and J. Branlund, “Thermal diffusivity of electrical insulators at high temperatures: evidence for diffusion of bulk phonon-polaritons at infrared frequencies augmenting phonon heat conduction”, J. Appl. Phys. 115, 163517 (2014).
    Xiaoli Tang and Jianjun Dong, "Lattice thermal conductivity of MgO at conditions of Earth's interior", Proc. Natl. Acad. Sci. U.S.A. 107, 4935-4954 (2010)

  • Understanding and Exploiting the Splendid Redox Physics of Ceria and It’s Derivatives

    Guest: Prof. Sossina M. Haile, Department of Materials Science and Engineering, Applied Physics Program and Department of Chemistry, Northwestern University, Evanston, IL
    Thursday, October 17, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Ceria and its derivatives find use in a wide variety of technologies from traditional applications in solid oxide fuel cells, catalysis, and electrochemical sensors, to new applications in computing, medicine, and water splitting. The suitability of ceria for these many applications derives in part from the redox flexibility of the material, with the predominantly Ce4+ ion adopting the 3+ oxidation state under conditions amenable to external control. The very high oxygen ion transport in suitably doped ceria is a second critical factor driving its technological value. Here we present recent results highlighting transport and redox activity in (i) bulk, (ii) grain boundary, and (iii) surface regions of ceria, obtained using a range of techniques from bulk thermogravimetric measurements, to in situ X-ray absorption studies and electron holography. We report on the unusual behavior of ceria upon substitution of Ce with the nominally isovalent species Zr. Contrary to what might be expected, Zr4+ has a dramatic impact on oxygen vacancy formation, in both the bulk and surface regions of the oxide. We further report on the dramatic role of trace impurities on transport across the grain boundaries of polycrystalline, rare-earth doped ceria. The insight garnered suggest new approaches to controlling material behavior for optimal technological characteristics.

  • Theoretical physics research with undergraduate students

    Guest: Prof. Robert C. Forrey, Distinguished Professor of Physics, Penn State University at Berks
    Thursday, October 10, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Theoretical physics research poses a challenge for engaging undergraduate student participation. Undergraduates usually have not developed the mathematical and computational skill sets needed to fully appreciate the complexities of the research. Nevertheless, the students are often able to make valuable contributions through data collection, analysis, and modeling. In the process, they develop foundational skills for their future while helping to advance the larger goals of the theoretical effort. Several recent examples in the context of molecular astrophysics will be reported in this talk. A rigorous theory of molecule formation will be described and related to the process of star formation. Machine learning methods which aim to provide state-resolved collision rates for modeling non-LTE astrophysical environments will be presented, and contributions from undergraduate students will be highlighted.

  • The PhD Skills --- A Personal View

    Guest: Prof. Yiping Zhao, Department of Physics and Astronomy, UGA
    Thursday, October 3, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    For graduate students and some undergraduate students, the pursuit of a PhD degree is a very important goal for their future career. This period could be one of the most precious/critical moments in their life for their skill development. Facing this changing and competitive modern world, what are the necessary skills that a graduate student needs to develop? I will give some of my personal thoughts on this subject combining our research on stroke treatment, and explain in detail how research is conducted. I hope this talk will be useful for both the graduate and undergraduate students who are interested in pursuing PhD degree.

    PhD Comic, "The Origin of the Theses"

  • In Search of Our Cosmic Origins

    Guest: Prof. Anthony Mezzacappa, Department of Physics and Astronomy, University of Tennessee
    Thursday, September 19, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Massive stars die in spectacular explosions known as core collapse supernovae, and their deaths give birth either directly or indirectly to the lion’s share of the elements in the Universe, the building blocks of life. Exactly how core collapse supernovae occur has been the focus of intense research for more than half a century. They are “multi-physics” events – in fact, much of classical and modern physics is manifest in these explosions. The modeling challenges, associated with capturing all of this physics, representing it mathematically for the purpose of performing computer simulations, and developing solution algorithms for the representative mathematical equations, optimized for today’s “leadership-class” supercomputers, are daunting. Yet significant progress has been made and progress is accelerating, due to the accumulation of knowledge from past simulation and the ever- increasing capabilities of supercomputers. I will discuss the physics and astronomy of core collapse supernovae, the current modeling state of the art, future modeling requirements, and the exciting prospects of a “multi-messenger” observation of a Galactic core collapse supernova in photons, neutrinos, and gravitational waves and what that could tell us about stellar death and about fundamental nuclear and particle physics that may never be accessible in terrestrial experiments.

     

    About Professor Anthony Messacappa

    Professor Anthony MezzacappaDr. Mezzacappa is the Newton W. and Wilma C. Thomas Endowed Chair in Theoretical and Computational Astrophysics in the Department of Physics and Astronomy at the University of Tennessee, Knoxville and the Director of the Joint Institute for Computational Sciences, joint between the University of Tennessee and its UT-Battelle partner universities and the Oak Ridge National Laboratory. Prior to this, Dr. Mezzacappa was a Corporate Fellow at the Oak Ridge National Laboratory, Group Leader for Theoretical Physics in its Physics Division, Group Leader for Computational Astrophysics in its Computer Science and Mathematics Division, and had been on staff at ORNL since 1996, where he created one of the leading core collapse supernova research programs in the world. Dr. Mezzacappa held postdoctoral appointments at the University of Pennsylvania and the University of North Carolina at Chapel Hill before joining ORNL. He completed his B.S. degree in physics at M.I.T. in 1980, and his Ph.D. in physics at the Center for Relativity at the University of Texas at Austin in 1988. He has worked in the areas of astrophysics and cosmology and specializes in the theory of core collapse supernovae.

    Dr. Mezzacappa received a DOE Young Scientist Award from Secretary of Energy Richardson and a Presidential Early Career Award for Scientists and Engineers (PECASE) from President Clinton in 1999 for his contributions to core collapse supernova theory. He was the Principal Investigator of the first large-scale, multi-investigator, multi-institutional computational astrophysics effort in the U.S. to focus on core collapse supernovae: the DOE SciDAC Terascale Supernova Initiative. Dr. Mezzacappa was elected a Fellow of the American Physical Society in 2004 and a UT-Battelle Corporate Fellow in 2005 in recognition of his supernova research and his role, much more broadly, in the development of computational science in the U.S.

    Dr. Mezzacappa was a member of the DOE Advanced Scientific Computing Advisory Committee’s Exascale Subcommittee and coauthor of several reports motivating tera-, peta-, and exa-scale computing, including Forefront Questions in Nuclear Science and the Role of High-Performance Computing; Scientific Challenges for Understanding the Quantum Universe and the Role of Computing at Extreme Scale; Modeling and Simulation at the Exascale for Energy and the Environment; andA Science-Based Case for Large-Scale Simulation. He was also a member of the Facilities, Funding, and Programs Independent Study Group for the Astro2010 Decadal Survey and a Co-Convener of the Computing Frontier, Astrophysics and Cosmology for the APS Divisions of Astrophysics and Particle Physics Snowmass 2013 High Energy Physics Decadal Planning. Dr. Mezzacappa also serves on the Editorial Board of the International Journal of High-Performance Computing Applications. Most recently, he is serving as Deputy Theory Chair of the Core Collapse Supernova Subgroup of the Gravitational Wave International Committee’s (GWIC) Third Generation (3G) Science Case Team, whose responsibility will be to develop The Science Case for the Next-Generation of Ground-Based Gravitational-Wave Detectors. Dr. Mezzacappa also serves as Chair of the international Supernova Multi-messenger Consortium, which he founded to assist the LIGO–Virgo Scientific Collaboration (LVC) in its efforts to detect gravitational waves from core collapse supernovae.

    Dr. Mezzacappa has authored or coauthored more than 200 scientific publications, has edited or coedited 8 volumes in his field or in the broader field of computational science, and has given numerous invited talks internationally. He has also devoted significant time and effort to education and outreach, both locally and nationally/internationally. He has been active in communicating science to the general public. He and his work have been featured on the National Geographic Channel and in Science, Physics Today, Scientific American, and HPCwire, to name a few venues.

  • The Time Domain and Multi-Messenger Astrophysics Frontiers-A brief sampling of personal explorations-

    Guest: Prof. Dieter Hartmann, Department of Physics and Astronomy, Clemson University
    Thursday, September 12, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    We live in a truly golden age of discovery-driven progress in deciphering the workings of our Universe. We are now collecting data across the entire electromagnetic spectrum, from the radio-band to gamma-rays. Light, the traditional tool of astronomers, has been augmented by other messengers, such as meteorites, energetic particles, neutrinos and gravitational waves. The era of Multi-Messsenger Astrophysics (MMA) is here and has enabled us to experience a breathtaking expansion of knowledge boundaries: we are in the era of precision cosmology, we discovered supermassive black holes in almost all galaxies, we mapped the shadow of a black hole, we listen to gravitational waves from merging compact star binaries, we discovered thousands of exoplanets, and we chart the cosmos back to the time of the first stars and galaxies. With focus on Time Domain astronomy (TDA), i.e., the study of transient phenomena, I will present current developments and discuss the road ahead. I will comment on the origin of gold, speculate on predictable discoveries and present the probe-class mission TAP (Transient Astrophysics Probe) presented to the ongoing Decadal Survey.

  • Attosecond X-Rays Reached the Water Window

    Guest: Prof. Zenghu Chang, CREOL, The College of Optics and Photonics, University of Central Florida
    Thursday, September 5, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    The advent of Ti:Sapphire lasers in the 1990s leads to the first demonstration for attosecond XUV pulses in 2001. In last 5 years, carrier-envelope phase stabilized lasers at 1.6 to 2.1 micron based on Optical Parametric Chirped Pulse Amplification pushed attosecond light sources to the “water window” X-rays that cover the 280 to 530 eV photon energy range, which enabled real-time observation of electron and nuclear motion in molecules containing carbon, nitrogen and oxygen. Very recently it was used to probe ionization, vibration and rotation dynamics of nitric oxide with unprecedented temporal resolution.

  • Nonequilibrium statistical mechanics: Small steps at a vast frontier of "pure and applied" theoretical physics

    Guest: Prof. Royce Zia, Department of Physics, Virginia Tech
    Thursday, August 29, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    When I was a student in the 60's, research in most physics departments are focused in elementary particles or solid state physics (some in astronomy, as in UGA). To prepare students for those frontiers, the core curriculum - which included equilibrium statistical mechanics - was adequate. Over the last decade or two, other disciplines blossomed onto the physics scene, from biophysics and econophysics to neuroscience and climate science. Most of the stochastic processes in these areas, on the other hand, take place under nonequilibrium conditions. Unfortunately, an overarching framework for such systems is far from being in place. Though many condensed matter theorists are engaged at this frontier, the general goal of understanding how complex collective behavior emerge from simple microscopic rules remains elusive. As an example of the difficulties we face, consider predicting the existence of a tree from an appropriate collection of H,C,O,N,... atoms! After a brief summary of the crucial differences between systems in thermal equilibrium (in standard text-books) and ones in nonequilibrium steady states, I will give a bird's-eye view of some key issues, ranging from the "fundamental" to the "applied." The methods used also span a wide spectrum, from computer simulations to stochastic field theoretic techniques. These will be illustrated in the context of an overview of my work, as well as one or two examples.

  • Spin Dynamics Under the Effect of Superparamagnetic Nanoparticles and Rashba Spin-Orbit Coupling

    Guest: Prof. Tho Nguyen, Department of Physics and Astronomy, UGA
    Thursday, August 22, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    Spintronics refers to the study of the role that electron spin plays in solid state physics and the possible devices that specifically exploit spin properties instead of, or in addition to, charge degrees of freedom. For practical applications, novel materials with long spin lifetimes and allowable spin coherent manipulation are paramount. In this colloquium, I will present (i) a novel way of manipulating electron spins in organic semiconductors (OSECs) by magnetic nanoparticles (MNPs)[1], and (ii) the role of Rashba spin-orbit coupling (SOC) in the optoelectronic and spin response in 2D and 3D organic/inorganic hybrid lead halide perovskite materials [2]. For the former, we demonstrated that Fe3O4 MNPs with superparamagnetic properties generate large magnetic dipole–dipole interaction with electron spins in OSECs. This interaction was found to be analogous to the effects of hyperfine interaction. Our study yields a new pathway for tuning OSECs’ magnetic functionality, which is essential for organic optoelectronic devices and magnetic sensor applications. For the latter, the presence of heavy Pb atoms in the perovskite crystal leads to a large intrinsic SOC, which, when combined with the breaking of the inversion symmetry, gives rise to giant Rashba-type SOC. In this part, I will show our preliminary results of Rashba SOC strength from 2D and 3D perovskites that depends on their structural inversion symmetry breaking, using several spintronic tools including magnetic field effects on conductivity, magnetic circular dichroism, and circular photoluminescence spectroscopy. Materials with strong Rashba SOC are particularly useful for electric-field controlled spin dynamics for future spin transistor logic devices.


    [1] Geng et al. DOI: 10.1039/c9mh00265k (2019)
    [2] Stranks et at. Nature Materials17, 381–382 (2018)

  • Photon Upconversion Sensitized by Bulk Lead-Halide Perovskite Films

    Guest: Prof. Lea Nienhaus, Department of Chemistry and Biochemistry, Florida State University
    Thursday, August 15, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    The sub-bandgap onset of rubrene-based organic light emitting diodes serves as an indicator of direct triplet exciton sensitization based on charge injection. Hence, materials which have a proper band alignment to allow for direct charge injection into the triplet state can enable a new path in sensitizing excitonic photon upconversion, while overcoming previous limitations resulting from poor exciton diffusion in nanocrystal-based systems. In particular, bulk lead halide perovskite (LHPs) thin films have emerged as efficient sensitizers for near-infrared-to-visible upconversion. The upconversion process is based on triplet-triplet annihilation (TTA) in the annihilator rubrene. Conservative estimates result in upconversion efficiencies upwards of 3%,[1] and upconversion has been shown to be efficient at incident powers comparable to the solar flux.

    Understanding the upconversion mechanism is crucial for the advancement of such devices. Our observations indicate that non-radiative trap filling in the LHP film and charge transfer to rubrene are likely competing pathways for the optically excited charge carriers. As a result, we obtain lower intensity thresholds Ith for efficient upconversion using thicker perovskite films, as these exhibit lower trap densities. However, a trade-off is observed: with increasing film thickness parasitic reabsorption of the singlets created by TTA also increases, which diminishes the visible light output of the device. [2]

    Two unexpected effects have been observed in perovskite-based upconversion devices: i) two rise times in the upconverted photoluminescence dynamics, and ii) a reversible ‘photobleach’ of the resulting upconverted emission. Both effects can be traced back to the existing triplet population level and the resulting population-dependent diffusion length, indicating that further optimization of the device is still needed. [3]

    [1] Nienhaus et al. ACS Energy Lett. 2019, 4, 888-895
    [2] Wieghold et al. Matter 2019, https://doi.org/10.1016/j.matt.2019.05.026
    [3] Wieghold et al. J. Phys. Chem. Lett. 2019, https://doi.org/10.1021/acs.jpclett.9b01526

  • From UGA to Beyond: Brown Dwarfs, Exoplanets, and Overhead Projectors

    Guest: Dr. Adam Schneider, Arizona State University
    Thursday, April 25, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    For this talk, I will recount some of my experiences as an undergraduate physics and astronomy student at the University of Georgia, including challenges I faced and (sometimes) overcame, and lessons learned from such experiences. I will also describe how my experiences as an undergraduate helped prepare for and shape some of my future career decisions. I will then describe some of my research accomplishments after my time as an undergraduate, and how I entered into the research world of brown dwarfs and exoplanets. I will then discuss in detail one of my current projects that I'm most passionate about -- the citizen science project BackyardWorlds.org -- which engages layperson volunteers to examine infrared images to look for nearby substellar objects in the vicinity of the Sun, and has the potential to discover new planets within our own solar system.

  • Quantum Sensing and Quantum Nanophotonics at Oak Ridge National Laboratory

    Guest: Dr. Benjamin J. Lawrie, Quantum Information Science, Oak Ridge National Laboratory
    Thursday, April 11, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    The Heisenberg uncertainty relationship for the intensity and phase of light describes a fundamental shot noise limit that cannot be surpassed with classical states of light. Two- mode squeezed light sources exhibiting continuous variable entanglement allow us to reduce the noise floor in optically transduced sensors that utilize a single quadrature of the optical field, enabling greater signal to noise ratios than are possible in the best possible classical sensors. I will present some of our recent results demonstrating quantum enhanced sensitivity for applications ranging from magnetometry to plasmonic sensing to atomic force microscopy. I will also discuss some of our recent research efforts exploring quantum nanophotonics with plasmonic nanostructures and single photon emitters in low-dimensional materials. I will close with a brief perspective on the potential for hybrid quantum systems incorporating both continuous-variable squeezed light sources and nanophotonic discrete-variable quantum light sources.

  • Correlated Nanoelectronics

    Guest: Prof. Jeremy Levy, Department of Physics and Astronomy, University of Pittsburgh. Pittsburgh Quantum Institute
    Thursday, April 4, 2019 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202)

    The study of strongly correlated electronic systems and the development of quantum transport in nanoelectronicdeviceshavefolloweddistinct,mostlynon-overlappingpaths. Electroniccorrelations of complex materials lead to emergent properties such as superconductivity, magnetism, and Mott insulator phases. Nanoelectronics generally starts with far simpler materials (e.g., carbon-based or semiconductors) and derives functionality from doping and spatial confinement to two or fewer spatial dimensions. In the last decade, these two fields have begun to overlap. The development of new growth techniques for complex oxides have enabled new families of heterostructures which can be electrostatically gated between insulating, ferromagnetic, conducting and superconducting phases. In my own research, we use a scanning probe to “write” and “erase” conducting nanostructures at the LaAlO3/SrTiO3 interface. The process is similar to that of an Etch-a-Sketch toy, but with a precision of two nanometers. A wide variety of nanoscale devices have already been demonstrated, including nanowires, nanoscale photodetectors, THz emitters and detectors, tunnel junctions, diodes, field-effect transistors, single-electron transistors, superconducting nanostructures and ballistic electron waveguides. These building blocks may form the basis for novel technologies, including a platform for complex-oxide-based quantum computation and quantum simulation.

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