Colloquium Spring 2010
This talk will describe ongoing work on a method -- Quasicontinuum Density Functional Theory -- to enable density functional theory calculations of defects in crystals. Defects determine critical properties of crystalline materials even though they occur at relatively low concentrations. They can interact over long distances through slowly decaying fields whose strength depends on the electronic structure of the core. Thus the study of defects requires electronic resolutions with continuum range. The main idea of the current method is a numerical discretization that adapts the resolution to the structure of the solution with no a priori ansatz or ad hoc patches. We demonstrate the idea with Orbital-free Density Functional Theory, highlight key properties through examples and describe ongoing work with DFT. Joint work with Vikram Gavini, Phanish Suryanarayana, Thomas Blesgen, Malena Espanol, Jarek Knap and Michael Ortiz
Martin Luther King, Jr. Day, University Holiday
Hydrogen bonds are ubiquitous in soft condensed matter systems and play a crucial role in basic chemical processes. Predicting accurately their behavior by molecular simulation is challenging because competing electronic structure mechanisms are present and quantum effects on the nuclear motion cannot be neglected. This talk will review recent progress in the field, focusing, in particular, on the combined role of electronic and nuclear quantum effects in condensed water phases.
The galactic frontier of our solar system is the vast region (100-200 AU from the Sun) where the supersonic flow of the solar wind collides with the surrounding interstellar medium. For many years experimental difficulties prevented exploration of this distant interstellar boundary of the heliosphere. The recently launched NASA's Interstellar Boundary Explorer (IBEX) has obtained the first global map of the heliosphere boundary in fluxes of energetic neutral atoms (ENAs). The mission has discovered the "ribbon", a highly-ordered region with the enhanced brightness of ENA emissions, not predicted by the existing models. (Science magazine featured the discovered ribbon on its cover in November 2009.) IBEX observations as well as in-situ measurements by the Voyager 1 and 2 spacecraft have challenged our established concepts of the heliosphere interaction. Mike Gruntman is Professor of Astronautics in the USC Viterbi School of Engineering
For more than ten years, ion-beam cancer therapy has been successfully used clinically in Germany and Japan. Proton-beam therapy is performed in many more centers around the globe. Thousands of patients per year are being treated. These therapies appear to be a more favorable alternative to the conventional photon therapy. Despite apparent experimental and clinical successes, a comprehensive theoretical description of a physical scenario is missing. One reason is that the phenomena initiated by an energetic ion incident on tissue happen on a variety scales in time, distance, and energy. I will present a multiscale approach to the physics involved in ion-beam therapy, starting from energetic ions incident on tissue and leading to cell death. This approach is phenomenological and I will discuss different effects, which are important on different scales.
Presidents' Day, University Holiday
I will discuss astrophysical observations that aim to uncover the nature of the dominant form of matter in the universe and the theoretical motivations behind these efforts.
Active plasmonic devices for on-chip nanophotonics
Luke A Sweatlock
Northrop Grumman Space Technology
Surface plasmon polaritons (SPPs) are a mode of electromagnetic excitation which occur at dielectric-metal interfaces, combining many of the propagation characteristics of optical waves with the nanoscale energy localization of electronics. Particular interest in plasmonics has been motivated by its promise as a transformative technology for chip-scale data communication. Plasmonic waveguides can be used to squeeze light into volumes smaller than the optical diffraction limit, serving as a bridge between high-bandwidth photonic networks and densely integrated, nanoscale electronic components. This talk will focus on devices which allow for dynamic control of SPPs, such as modulators and color filters. We will compare several experimentally demonstrated materials systems for active plasmonics, which include the electro-optic effect in barium titanate; thermo-optic solid state phase change in vanadium oxide; and field effect modulation in metal-oxide-silicon waveguides. Such devices are not only orders of magnitude in smaller in size than corresponding photonic devices; the nanoscale active volume results in proportionally smaller intrinsic energy consumption as well. Finally, we will discuss the opportunity to use numerical optimization tools to overcome remaining design challenges, and achieve integration of active plasmonic devices into functional nanophotonic networks.
Mapping the Sky in Infrared Light with the Wide-field Infrared Survey Explorer
Deputy Project Scientist for the Wide-field Infrared Survey Explorer, Jet Propulsion Laboratory
The Wide-field Infrared Survey Explorer (WISE) is a NASA space telescope that launched on December 14, 2009. WISE consists of a 40 cm telescope that will image the entire sky in four infrared wavelengths: 3.4, 4.6, 12 and 22 microns. The mission will survey the entire sky 1.5 times in nine months by continuously observing every 11 seconds, producing ~50-60 GB of data per day. Among its goals are finding the nearest stars to the Sun, the most luminous galaxies in the entire universe, and the Earth's nearest neighbors, the asteroids and comets. The last time the whole sky was mapped at these wavelengths was in 1983, so new surprises are sure to come from this mission.
Immobilized Biocatalysts for Enzymatic and Microbial Fuel Cells
Microbiology & Applied Biochemistry, Air Force Research Laboratory
Biological fuel cells provide electricity by breaking down substrates to release the energy stored in chemical bonds. Unlike conventional fuel cells, the precious metal catalysts are replaced by enzymes or microbes for fuel oxidation in the anode, and potentially for oxygen reduction in the cathode. The catalytic breadth of enzymes allows the use of energy dense substrates including carbohydrates and macromolecules as fuel sources. In order for the released energy to be captured, the biocatalysts must be electronically linked to the anode and cathode. One step toward that link is to fix the catalysts into close associations with the electrode surfaces. The approach intends to build a hierarchical, catalytic architecture that is amenable to practical systems. Contemporary tools in nanotechnology, biomimetic assembly, and materials chemistry provide many methods to design and create the biocomposite materials. Our laboratory applies combinations of these tools for materials development and characterization, along with assessment of the biochemical activity. Recent in-house experimental results provide systematic advances in enzymatic fuel cell technology and address materials approaches that achieve direct electron transfer between enzyme redox cofactors and electrode surfaces.
Many animals, including migratory and non-migratory birds derive directional information from the geomagnetic field to orient their movements. This magnetic compass has been well characterized behaviorally, but the underlying reception mechanism has remained enigmatic. For many years, the use of biological magnetic materials such as magnetite crystals, had been assumed to underlie biological compasses. Recently, the suggestion that magnetically sensitive photochemical radical-pair reactions can provide a basis for magnetic sensitivity has received renewed attention. The radical pair model proposes a role of spin-selective processes as the magnetically sensitive step, suggesting a role of quantum physics in a key biological response. We will review behavioral, spin-chemical, and neurobiological evidence from our and other groups supporting this suggestion. Finally, we will discuss future avenues of research towards identifying not only the mechanism, but also the chemical nature of the receptors underlying magnetoreception, and in particular the photoreceptor chryptochrome, an emerging candidate for the long sought after magnetoreceptor.
Ritz, T., P. Thalau, J. Phillips, R. Wiltschko, W. Wiltschko. Resonance effects indicate a radical pair mechanism for avian magnetic compass. Nature 429:177 (2004).
Ritz, T., M.Ahmad, H. Mouritsen, R. Wiltschko, W. Wiltschko Photoreceptor-based magnetoreception: optimal design of receptor molecules, cells, and neuronal processing. J. R. Soc. Interface 7:S135-S146 (2010)
I will discuss the general problem of strong correlations. In particular the often suggested passage to the extreme limit of infinite on site interaction, changes the rules of the game. It requires us to solve field theories with non canonical objects (i.e. objects that are neither Bosons or Fermions). In particular the main bottleneck, namely the absence of the Wick theorem, has prevented its solution, due to the absence of perturbative Feynman diagram schemes.
We have discovered a recent method, using the ``Schwinger way'' instead, that circumvents the absence of the Wick theorem, and leads to a suitable Schwinger Dyson hierarchy of equations. Preliminary results show that the resulting Extremely Correlated Quantum Liquid (ECQL)state that emerges, violates the Luttinger Ward volume theorem for the Fermi surface.
Investigating Dark Matter with the Fermi Large Area Telescope
SLAC, Stanford University
The Fermi Large Area Telescope (LAT) has been successfully launched from Cape Canaveral on 11 June 2008. It is exploring the gamma ray sky in the energy range from 20 MeV to over 300 GeV with unprecedeted sensitivity. One of the most exciting science questions that the Fermi LAT will address is the nature of dark matter. Several theoretical models have been proposed that predict the existence of Weakly Interacting Massive Particles (WIMPs) that are excellent dark matter candidates. The Fermi LAT investigates the existence of WIMPs indirectly, primarily through their annihilation or decay into photons and into electrons and positrons. Recent results on these searches will be presented.
Much of our understanding of modern cosmology is based on the "concordance model" which uses a handful of free parameters to account for the overall properties of the universe, from the big bang to the present day. I will review the concordance model and its implications for the history of the universe, and show how its free parameters are related to fundamental physical processes that we do not understand. I will focus on the subset of parameters which are tied to inflation: a phase of accelerated expansion thought to have occurred moments after the big bang. I will then discuss further observables -- e.g. gravitational waves and departures from Gaussianity by the primordial density perturbations -- that might one day be added to the inflationary sector of the concordance parameter set. Most inflationary models predict that most of these observables are very small. Consequently, these observables constitute fingerprints of specific inflationary scenarios, and provide windows into both the first moments of the universe's existence and fundamental physics at the highest imaginable energies.