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Physics Colloquium

 

Spring 2016 Colloquium Schedule

Date Speaker Title Host
January 25 Mu-Chun Chen
UC Irvine
Neutrinos and Physics beyond the Standard Model Yuri Kamyshkov
February 1 Jeremy C. Smith
UT, BCMB Department
Protein Physics and Drugs Jaan Mannik
February 8 Peter J. Hirschfeld
University of Florida
Disorder and Quasiparticle Interference in High-Tc Superconductors Steve Johnston
February 15 Ruxandra I. Dima
University of Cincinnati Department of Chemistry
Multi-scale Modeling of the Nanomechanics of Biomolecular Shells Jaan Mannik
February 22 Sergei Urazhdin
Emory University
Active Magnetic Nanostructures Driven by Spin Currents Jian Liu
February 29 John F. Mitchell Argonne National Lab What's Cooking? Haidong Zhou
March 7 James C. Weisshaar
U. Wisconsin-Madison
Spatiotemporal Organization of the E. coli Cytoplasm Jaan Mannik
March 14 Spring Break NA NA
March 21 Matteo Rini
Deputy Editor, Physics
Science Communication: Take Charge of It!

Norman Mannella
March 28 Zhigang Jiang
Georgia Tech
Magnetoplasmons and Magnetophonons in Graphene and Graphite Haidong Zhou
April 4 David Hsieh
Caltech
Revealing Hidden Phases in Correlated Electron Systems using Nonlinear Optics Jian Liu
April 11 Brent VanDevender
Pacific Northwest National Lab
Weighing the Neutrino Nadia Fomin
April 18 Dean J. Lee
North Carolina State University
Lattice Simulations of Nuclei Lucas Platter
April 25 Honors Day Celebration Student Awards and Presentation of Distinguished Alumni Award to Dr. Glenn Young  

Abstracts

January 25, 2016

Neutrinos and Physics beyond the Standard Model
Mu-Chun Chen
University of California, Irvine

Neutrino, having several Nobel Prizes in Physics directly under its belt, including this year's Prize for the discovery of neutrino mass by the Super-Kamiokande and Sudbury Neutrino Observatory Collaborations, is the most elusive and, besides photon, the most abundant particle in the Universe. On the other hand, there are still many outstanding questions about the neutrino that are yet to be answered. While we now know that the Higgs particle gives masses to all charged elementary fermions, we still have no clue what generates the neutrino masses. Furthermore, we don't know at the fundamental level what causes neutrinos to morph from one type to another while traveling through space, and whether the time reversal symmetry is broken in neutrino oscillation. Neutrinos also play a very important role in cosmology. While the Cosmic Microwave Background takes us all the way back to 380,000 years after the Big Bang, the standard Cosmic Neutrino Background, if ever observed, will take us all the way back to the very first second after the Big Bang! In addition, neutrinos may be responsible for the generation of the matter-antimatter asymmetry in the Universe.

In this talk, I will briefly review the current state of neutrino physics. I will discuss possible new physics, based on symmetry principle, that can naturally give rise to small neutrino masses and their oscillation pattern. Because of the symmetry, there exist predictions that allow for the theory to be tested at current and upcoming experiments. I will present a novel mechanism for the violation of time reversal symmetry, which is one of the necessary conditions for generating dynamically the matter-antimatter asymmetry in the Universe. Finally, I will elucidate a tantalizing possibility of the existence of a non-standard Cosmic Neutrino Background, enabled if neutrinos are Dirac fermions, that would provide a novel window into the very early Universe.


February 1, 2016

Protein Physics and Drugs
Jeremy C. Smith
Governor’s Chair, University of Tennessee and Director
Center for Molecular Biophysics, Oak Ridge National Laboratory

Proteins are nature’s most finely-tuned materials, and the source of great mysteries. Energy landscapes for functionally-important internal protein motions are highly complex, with effective dynamical relaxation times existing over many decades in time, from ps up to ~100 s. Our recent work using massive supercomputer simulations has shown that motions in single protein molecules are non-ergodic, non-equilibrium and exhibit ageing, properties arising from the fractal nature of the topology and geometry of the energy landscape explored. However, although motions in proteins are complex, dynamic concepts can accelerate drug discovery, and we give practical examples of this in the discovery of lead compounds for several diseases.


February 8, 2016

Disorder and Quasiparticle Interference in High-Tc Superconductors
Peter Hirschfeld
Department of Physics, University of Florida

New superconductors are discovered every year, but why the electrons form Cooper pairs is still a mystery in many cases. Often understanding the symmetry and structure of the pair wave function provides important clues to the answer, but direct probes are difficult to implement. In recent years, creative use has been made of the information available from scanning tunneling microscopy experiments (STM) on superconducting surfaces to determine the electronic energy bands of the materials and the momentum dependence of the pair wave function. Crucial to this so-called "quasiparticle interference" (QPI) experiment is the understanding of the role of disorder, which creates ripples in the Fermi sea. I will discuss the theory of QPI, give some examples where experiments have proven crucial, and propose some new methods to tighten the interplay of experiment and theory, working toward the solution of the high-Tc problem.


February 15, 2016

Multi-scale Modeling of the Nanomechanics of Biomolecular Shells
Ruxandra I. Dima
Department of Chemistry, University of Cincinnati

Large-size biomolecular systems that assemble, disassemble, and self-repair by controlled inputs play fundamental roles in biology. Microtubules are important in cytoskeletal support and cell motility. Physical properties of capsids of plant and animal viruses are important factors in capsid self-assembly, survival of viruses in the extracellular environment, and their cell infectivity. We focus on deciphering the microscopic origin of the physico-chemical properties of such biological assemblies and the molecular mechanisms of their response to controlled mechanical inputs. Because assemblies have modular architecture and strong interand intra-molecular coupling that modulate their properties, any approach has to model them on multiple spatial scales. We developed a multi-scale approach, combining coarsegraining1,2,3 with atomic details3,4, implemented on Graphics Processing Units (GPUs) for computational acceleration, to map out the mechanical properties of large size biological systems on experimental timescales. I will present our results for the micromechanics of microtubules4,5,6,7, related to the mechanism of microtubule disassembly, and our findings regarding the link between discrete microscopic transitions and the continuous mechanical response of the Cowpea Chlorotic Mottle Virus capsid at the macroscopic level8, in direct correspondence with AFM indentation experiments.


February 22, 2016

Active Magnetic Nanostructures Driven by Spin Currents
Sergei Urazhdin
Department of Physics, Emory University

21st century has witnessed a dramatic transformation of magnetism research from passive structures to electronically driven magnetic (spintronic) nanostructures that can find applications in memory and microwave technologies. It has been predicted that such devices are most efficient when operated by pure spin currents – spin flows not associated with charge currents. I will describe our recent demonstrations of two types of spin current-driven active nanodevices, one based on the spin Hall effect, and another based on nonlocal spin injection. I will show that the operation of these nanodevices generally relies on the nonlinear effects that result in the transformation of the dynamical spectrum of the magnetic system under the influence of spin current. The most interesting and potentially useful behaviors occur when one dynamical mode becomes singled out from the spectrum. I will describe two experimentally observed scenarios: one can be qualitatively understood in terms of the quasi-equilibrium statistics of Bose particles and is similar to the Bose-Einstein condensation, another can be understood in terms of the nonlinear Schrödinger equation and is similar to the energy quantization for a particle in a box.

Spin-current oscillators can be utilized, among other applications, as local sources in magnonic circuits – circuits that utilize spin-waves (magnons) as the information carrier. If time permits, I will describe a novel approach to the integration of spin-current nano-oscillators with magnonic waveguides based on the effects of the dipolar fields in magnetic nano-patterns. The approach enables good spectral matching between the localized oscillation and the magnonic waveguide, and efficient directional transmission of spin waves excited by the spin current. These results facilitate the development of electronically controllable magnetic nanocircuits that integrate information storage, transmission, and manipulation.


February 29, 2016

What’s Cooking?
John Mitchell
Argonne National Laboratory

Correlated electron oxides remain a foremost area of research, principally because they host essentially the entirety of contemporary condensed matter science — superconductivity, magnetism, multiferroicity, topological protection, etc. In this talk I will discuss some recent work in a few areas of TMO materials physics: Specifically, I will describe our recent work on iridates, in particular magnetic and electronic properties of the single layer Sr2IrO4—implicated as a potential analog to cuprate superconductors, and the quantum spin liquid candidate Na2IrO3. This will include some discussion of our attempts to create related phases. I will then diverge to discuss some of the new materials we are currently working on that have become accessible to us through high pressure floating zone growth, focusing on a new class of nickelates with charge stripes. The hope is to give an appreciation of the kinds of materials that might be newly available as a consequence of this technique, to provide a view on the materials strategies of interest to our group and hopefully to pique the interest of potential collaborators at UTK.


March 7, 2016

Spatiotemporal Organization of the E. coli Cytoplasm
James C. Weisshaar
Department of Chemistry, University of Wisconsin-Madison

Superresolution fluorescence microscopy has enabled us to locate and track single ribosomes (chromosomally expressed S2-mEos2), RNA polymerase copies (chromosomally expressed ’-mEos2), and DNA loci (ParB-XFP labeling) in live E. coli with spatial accuracy of  ~ 30 nm and time resolution of 2-10 ms when needed. Ribosome-RNAP segregation is strong, arguing against co-transcriptional translation as the primary means of protein synthesis. Diffusion of both ribosomes and RNAP is heterogeneous. This enables us to distinguish translating 70S-polysomes from 30S subunits searching for translation initiation sites. We can also distinguish transcribing RNAP copies from those searching for transcription initiation sites. Time-dependent imaging of the DNA stain Sytox Orange after drug treatment indicates that on the 0-5 min timescale, both rifampicin and chloramphenicol induce nucleoid contraction. This corroborates the transertion hypothesis (co-transcriptional translation and simultaneous insertion of membrane proteins). The combination of these new experimental data with coarse-grained models of DNA-ribosome mixing suggests a picture in which expansion of the nucleoid by transertion is important for optimal cell function. The expanded nucleoid enables facile recycling of ribosomal subunits from ribosome-rich regions (where most translation occurs) to the nucleoids (where they can initiate co-transcriptional translation). At the same time, free polysomes are excluded from the nucleoids. The resulting spatial segregation may enhance overall growth rate by restricting the space within which RNAP searches for transcription initiation sites and ribosomal subunits search for translation initiation sites. This in turn may enhance overall growth rate.


March 21, 2016

Science Communication: Take Charge of It!
Matteo Rimi
Deputy Editor,
Physics

Scientists have a responsibility to share the meaning and implications of their work, but receive little training in communication, and often feel unprepared to communicate with the public, the media and even researchers from other fields. In this talk, I will discuss how our journal Physics (https://physics.aps.org/) strives to communicate research to a broad audience and share some general thoughts on science communication derived from my experience as a writer, editor, press officer and scientific consultant to policy makers.


March 28, 2016

Magnetoplasmons and Magnetophonons in Graphene and Graphite
Zhigang Jiang
School of Physics, Georgia Institute of Technology

Probing energy, symmetry and dispersion of low-lying excitations (e.g., electrons, holes, plasmons, phonons) in solids and studying many-body interactions among them and with light are focal points of our research. Here we first report on the observation of plasmon-type collective excitations in quasi-neutral graphene nanoribbons exposed to a perpendicular magnetic field. Most saliently, we reveal a peculiar scaling behavior which allows us to identify this mode with the upper-hybrid mode (UHM) between the plasmon resonance and the lowest Landau level (LLL) transition in graphene. This scaling is different from that of the UHM in conventional two-dimensional electron gases with parabolic bands or in highly doped graphene as well as from that of magnetoexcitons. Second, we show that similar hybrid modes between the K-point optical phonons in graphite and the LLL transition may form when the inter-LL transition energy is tuned to cross the optical phonon energy by varying the magnetic field. In this case, a marked avoided-level-crossing splitting of the cyclotron resonance is observed, and the strength of electron-phonon interactions is obtained from the amplitude of the splitting.


April 4, 2016

Revealing Hidden Phases in Correlated Electron Systems using Nonlinear Optics
David Hsieh
Institute for Quantum Information and Matter, California Institute of Technology

The iridium oxide family of correlated electron systems is predicted to host a variety of exotic electronic phases owing to a unique interplay of strong electron-electron interactions and spin-orbit coupling. There is particular interest in the perovskite iridate Sr2IrO4 due to its striking structural and electronic similarities to the parent compound of high-Tc cuprates La2CuO4. Recent observations of Fermi arcs with a pseudogap behavior in doped Sr2IrO4 and the emergence of a d-wave gap at low temperatures further strengthen their phenomenological parallels. In this talk I will describe our recently developed nonlinear optical spectroscopy and wide field microscopy techniques, which are highly sensitive to both the lattice and electronic symmetries of crystals. I will present results on the Sr2IrO4 system that reveal a subtle structural distortion and a hidden electronic phase that have previously eluded other experimental probes. I will comment on its relevance to the pseudogap region and also draw comparisons with our recent nonlinear optical data on the cuprates.


April 11, 2016

Weighing the Neutrino
Brent VanDevender
Pacific Northwest National Laboratory

The Standard Model of Particle Physics is constructed with massless neutrinos. However, neutrino flavor oscillation, the discovery of which earned the 2015 Nobel Prize, is definitive evidence for non-zero neutrino mass. Understanding the absolute scale and theoretical mechanism of neutrino mass appears to be of fundamental significance for physics beyond the Standard Model. Neutrinos left over as relics of the Big Bang are the second most numerous particle in the Universe, after photons. They might have affected the evolution of large scale gravitational structure in the Universe by their sheer abundance, despite being extremely light, and consistent with current upper limits on their mass. We will review the physics of neutrino mass and its role in particle physics and cosmology. We will then discuss the KATRIN and Project 8 experiments that will attempt to measure neutrino mass by the so-called tritium endpoint method.


April 18, 2016

Lattice Simulations of Nuclei
Dean Lee
North Carolina State University

This talk reviews some recent progress made by the Nuclear Lattice Effective Field Theory Collaboration. In the first part I discuss an ab initio calculation of alpha-alpha scattering which uses a technique called the adiabatic projection method. In the second part I present evidence that nuclear matter is near a quantum phase transition. I discuss the control parameter for this transition and the implications for nuclear clustering and the binding of protons and neutrons within nuclei.


Previous Physics Department Colloquia



 

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