Colloquium
Unless otherwise noted, the physics colloquia are held in Room 307 of the Science and Engineering Research Facility. Refreshments are served at 3:00 p.m. with the talk following at 3:30.
Karen Lloyd, University of Tennessee
A Thermodynamic Quirk in Marine Sediments May Explain Why We're Not All Suffocating in Methane Right Now
Most of the microbes buried in marine sediments have never been cultured in a laboratory, so we must design experiments to test their physiology without removing them from their natural settings. A type of archaea, called ANME-1 have been shown to oxidize methane anaerobically in a consortium with sulfate reducing bacteria through a mechanism that has not been fully characterized. However, since ANME-1 survive on very low energies, theory predicts that they should reverse their metabolism to methanogenesis when the reaction becomes exergonic in that direction. We found evidence that this occurs in the White Oak River estuary, and a re-analysis of some published literature supports our conclusions. This nimble switching between methanogenesis and anaerobic methane oxidation in natural settings suggests that, despite being relatively slow growers, these organisms are well-poised to adapt their metabolism quickly to environmental changes. The fact that they get energy from both the forward and reverse directions of a single metabolism, depending on the exergonicity of the chemical reaction, may explain how they are able to consume 99% of the vast quantity of methane that is microbially produced in marine sediments.
Lucas Platter, University of Tennessee
Effective Field Theories for Electroweak Physics in Nuclear Physics
Effective field theories have led to a revolution in nuclear physics. They are facilitating calculations based on first principles and with quantifiable uncertainties. Electroweak processes provide a unique way of testing novel approaches in nuclear physics, I will discuss recent progress in the calculation of electroweak processes in the few-nucleon sector. In particular, electroweak capture reactions that also involve the Coulomb interaction are hard to measure experimentally since the cross section is exponentially suppressed due to the Coulomb repulsion. SI will focus on proton-proton fusion the initial reaction that starts the proton-proton chain reaction network that is generating energy in the sun. I will also address how this reaction is related to muon capture on the deuteron, an experimentally measurable process. For both of these processes, I will also illustrate different methods to quantify the uncertainties of our predictions. I will also discuss our recent progress in calculate the decay rate for beta-delayed proton decay of Beryllium-11.
Royce Zia, Virginia Tech
Non-equilibrium Statistical Mechanics: a Growing Frontier of "Pure and Applied" Theoretical Physics
Founded over a century ago, statistical mechanics for systems in thermal equilibrium has been so successful that, nowadays, it forms part of our physics core curriculum. On the other hand, most of "real life" phenomena occur under non-equilibrium conditions. Unfortunately, statistical mechanics for such systems is far from being well established. The goal of understanding complex collective behavior from simple microscopic rules (for how the system evolves, say) 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! Over the last three decades, an increasing number of condensed matter theorists are devoting their efforts to this frontier. After a brief summary of the crucial differences between text-book equilibrium statistical mechanics and non-equilibrium statistical mechanics, 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 simple computer simulations to sophisticated field theoretic techniques. These will be illustrated in the context of an overview of our work, as well as one or two concrete examples.
Douglas Higinbotham, Jefferson lab
The Proton Radius Puzzle
For many years scientists believed that the proton radius was 0.877(6) fm based on a series of atomic Lamb shift and electron scattering measurements. In 2010, a new type of measurement, making use of muonic hydrogen, determined the radius to be 0.842(1) fm. The large systematic difference between muonic hydrogen measurements and the previous results has become known as the proton radius puzzle. To solve this puzzle a world-wide theoretical and experimental has been undertaken. I will review the current status of puzzle with an emphasis on the latest experimental and theoretical results.
Lance Cooper, University of Illinois Urbana-Champaign
Shining Light on Collective Excitations in Strongly Interacting Materials
Strongly interacting condensed matter systems are characterized by a diverse range of phases and phenomena that can often be controlled via sophisticated device fabrication or external perturbations, e.g., magnetic field, pressure, etc. Perhaps the most interesting characteristic of the many phases exhibited by interacting materials are their distinctive collective excitations, including phonons, magnons, polarons, magnetic monopoles, Higgs modes, etc. In this talk, I'll discuss some of the collective excitations exhibited by various states of matter and describe some of the techniques useful for probing these excitations. I'll then focus on some of our group's work using pressure- and magnetic-field-tuned light scattering from magnons, phonons, and hybrid collective modes to elucidate the microscopic properties of and multifunctional behaviors in magnetically frustrated and multiferroic materials.
Sung-Kwan Mo, Lawrence Berkeley National Laboratory
Less in Different
Atomically thin two-dimensional (2D) materials often exhibit novel physical properties that are largely different from their bulk counterparts. These properties are governed by the changes in the band structure and the lattice symmetry, and most pronounced in their single layer limit. Angle‐resolved photoemission spectroscopy (ARPES) is a direct tool to investigate the underlying changes of band structure to provide essential information for understanding and controlling such properties. In this talk, I will first review some of the recent developments in 2D materials research. Then, I will introduce our approach of investigating the electronic structures of 2D layers, combining bottom-up growth using molecular beam epitaxy (MBE), in situ ARPES, and scanning tunneling microscopy (STM). Distinct electronic and topological properties of these 2D films, such as indirect-direct band gap transition, gigantic exciton binding energy, persistent charge density wave order, and quantum spin Hall insulator phase, will be discussed.
Andrew Steiner, UT Physics
Determining the Properties of Dense Matter from Neutron Star Observations
Neutron stars are a unique laboratory for nuclear physics. In this talk, I will show how neutron star observations provide unique insights into quantum chromodynamics (QCD), and the interactions between neutrons and protons. For example, neutron stars have the fastest sound waves in the universe, exceeding the naive expectations which one might have from lattice QCD. I present our predictions for the tidal deformability, (aka "squishiness") of neutron stars, and show how that prediction was verified in the observation of a double neutron star merger, GW 170817, by the Laser Interferometer Gravitational Wave Observatory (LIGO). Finally, I show how to go beyond the equation of state to obtain information about the composition and superfluid properties of dense matter. For example, neutron superfluidity pervades the star, preventing beta decay at almost all densities.
Erik van Heumen, University of Amsterdam
Putting Holographic Theories for Strange Metals to the Test: the Latest Reports from the Lab
The development of AdS/CFT as a toolbox to understand strongly correlated electron systems has taken significant progress in recent years. In contrast to string theory, there is one important difference that allows the application of AdS/CFT to condensed matter systems to blossom: experiments! It is only fitting that the photo-electric effect that earned Einstein his Nobel prize, enables us to test predictions coming out of the theory for which he is best known. The experiment is known as angle-resolved photoemission spectroscopy, or ARPES and is widely used to directly measure the properties of dressed quasiparticles in solids. What AdS/CFT predicts for the strange metal phase is that we should see a breakdown of the quasiparticle concept that has been at the heart of our understanding of metals for nearly a century. To achieve this goal demands that we push this experiment to its limits and in this talk, I will show how far we have gotten with this.
Manas Mukherjee, National University of Singapore
Trapped Ion—One Device Many Applications from Quantum Computing to Quantum Sensing
Ion trap has emerged as a device of choice for numerous applications due to its ability to isolate, hold and manipulate individual atoms. This brings in the opportunity to control quantum states of these atoms with high precision, thus becoming one of the leading technologies in quantum computing, atomic clocks and other metrology applications. Starting with a brief overview of ion trapping and laser cooling, its application towards quantum information processing will be discussed. A comparative study of ion trap devices with other competing technologies will show the challenges in developing large scale quantum computers. On the other hand, the opportunities that lies in using the Noisy Intermediate-Scale Quantum (NISQ) devices to solve useful problems that are beyond the reach of any classical computing will mark the start of a new era in computing. Towards the end a few other application of trapped ions, particularly in quantum simulation and sensing will be touched upon. The colloquium will be introductory in nature while showing some of the recent developments in the field like achieving quantum supremacy and the race to achieve it.
K C Huang, Stanford University
Model Systems for Microbial Ecology
The past decade has seen an explosion in characterization of how microbial communities impact human health and the environment. Yet, our understanding of the molecular mechanisms driving community assembly, dynamics, and resilience has lagged far behind. I will discuss two of our efforts to develop "model communities" for microbiome research. First, we use the simple bacterial community in fly guts to uncover connections between growth, pH, and antibiotic sensitivity. Second, we use a top-down approach to derive stable, complex communities from human stool that exhibit colonization resistance to pathogens, robustness to antibiotic perturbation, and stability over months of passaging. I will end with a discussion of how these approaches can impact studies of other microbial ecosystems.
Megan Connors, Georgia State University
Probing the Quark Gluon Plasma with Jets at RHIC
The Relativistic Heavy Ion Collider (RHIC) collides gold ions at high energies to create a hot and dense environment in which a quark gluon plasma (QGP) is formed. The QGP enables us to study phenomena unique to the nuclear strong force and advance our understanding of Quantum Chromodynamics (QCD). High momentum particles produced in the early stage of the collision lose energy as they traverse the QGP before fragmenting into a collimated sprays of hadrons known as jets. Jets have proven to be a useful probe of the QGP and can be measured by clustering particles with jet finding algorithms or via two particle angular correlations. This talk will present the physics knowledge gained from current jet measurements including the latest results from two particle correlation studies in PHENIX as well as the status of the state-of-the-art jet detector, sPHENIX, which is scheduled to start collecting data at RHIC in 2023.