Spring 2005

February 2, 2005
Dr. Roya Zandi
UCLA

Title: Origin of icosahedral symmetry in viruses

Viruses infect all kinds of hosts (bacteria, plants, and animals), and with all degrees of severity (from the common cold to AIDS). Most of these viruses involve a spherical shell (capsid) which protects their genome. Amazingly enough, despite the tremendous diversity in the protein "building blocks" of these capsids, the structures they adopt almost always have icosahedral symmetry. In this talk, I show that this striking feature is a consequence of a universal physical principle of energy minimization. I present a simple model which accounts for the different types of observed icosahedral structures, as well as special exceptions.



February 9, 2005
Nuh Gedik
Arthur Amos Noyes Laboratory of Chemical Physics
California Institute of Technology
www.its.caltech.edu/~gedik

Title: Tracking the electrons in high temperature superconductors

Tremendous progress in the technology of short-pulse lasers has created opportunities to study condensed matter systems in a novel way. Among the most interesting of these systems are high-transition temperature (Tc) superconductors. Despite well over a decade of intense research, the origin of superconductivity in these complicated materials is still unknown. After a brief overview of the history of superconductivity, I will discuss our recent measurements of nonequilibrium dynamics in these materials performed by using "ultrafast" laser techniques. I will present an overview of our progress to date and discuss how ultrafast methods may, in the near future, make a decisive contribution to uncovering the mechanism of superconductivity in these materials.



February 16, 2005
Jiong Qiu, Big Bear Solar Observatory

Title: Magnetic Reconnection in Solar Eruptions

Nearly once a day a large bundle of plasmas wrapped in magnetic fields, often called a flux rope, is violently hurled out of the Sun to the interplanetary space, which may reach the Earth's magnetosphere and cause magnetic storms. This is coronal mass ejection (CME), the most spectacular energy release on the Sun. CMEs are believed to be driven magnetically, though the exact mechanism remains in heated debate. Many CMEs are accompanied by solar flares, characterized by sudden brightening across a wide range of electric-magnetic spectrum on the Sun's surface, which lasts from minutes to hours. It is believed that vehement energy release in solar flares is governed by magnetic reconnection, when anti-parallel magnetic field lines collide and reconnect with each other, efficiently converting magnetic energy into heating plasmas and accelerating charged particles. Scientists have explored the relationship between flares and CMEs and recently discovered a close temporal correlation between the rising motion of CMEs at the early stage of eruption and impulsive emissions of solar flares in a number of CME-flare events. We have furthered such studies by linking flares and CMEs with magnetic reconnection. The rate of magnetic reconnection is evaluated from observations based on theoretical framework, and it is shown that both acceleration of mass ejections and energetic emission of solar flares are temporally correlated with magnetic reconnection rate. The magnitude of mass acceleration is also found to be scaled with magnetic reconnection rate. These findings strongly argue for the important role of magnetic reconnection in the kinematical evolution of CMEs during the early phase of eruption, when acceleration is most significant.



March 30, 2005
Demetrius J. Margaziotis
Department of Physics and Astronomy
California State University, Los Angeles

Title: What Is Going On With The Proton?

The electromagnetic structure of the proton (and neutron) is of fundamental importance in nuclear physics. Of particular interest is the distribution of electric charge and magnetization. These quantities are normally described in terms of "form factors" and they have been studied for over 50 years. The recent availability of highly polarized, high energy and intensity electron beams from the CEBAF accelerator at the Thomas Jefferson National Accelerator Facility in Newport News, VA has prompted a series of new form factor measurements for the proton using new and more accurate techniques than in the past. Some surprises have emerged. The results of these measurements and how well they agree (or don't agree) with previous data and with theoretical predictions will be discussed.



April 6, 2005
Michael Zwolak
Institute for Quantum Information
California Institute of Technology

Title: Charge Transport in DNA: the pi-way or the highway?

Within the DNA double-helix there is a neatly stacked array of bases. That this array may form a pi-channel, or pi-way, for charge transport was suggested over 40 years ago. Recently, research into the possibility that DNA might be a molecular wire has grown tremendously due to its potential use in molecular electronics and the relevance of charge transport to biological reactions. I will discuss fundamental issues having to do with charge transport in DNA and also how we may be able to sequence DNA by exploiting the different electronic characteristics of the DNA bases.



April 13, 2005
Merav Opher
Jet Propulsion Laboratory
butch.engin.umich.edu/~merav

Title: Effects of a Tilted Heliospheric Current Sheet at the Edge of the Solar System

Recent observations indicate that Voyager 1, now beyond 90 AU, is in a region unlike any encountered in it's 26 years of exploration. There is currently a controversy as to whether Voyager 1 has already crossed the Termination Shock, the first boundary of the Heliosphere. An important aspect of this controversy is our poor understanding of this region. The region between the Termination Shock and the Heliopause, the Helisheath, is one of the most unknown regions theoretically. In the Heliosheath magnetic effects are crucial, as the solar magnetic field is compressed at the Termination Shock by the slowing flow. Therefore, to accurately model the heliosheath the inclusion of the solar magnetic field is crucial. Recently, our simulations showed that the Heliosheath presents remarkable dynamics, with turbulent flows and a presence of a jet flow at the current sheet that is unstable ue to magnetohydrodynamic instabilities. We showed that to capture these phenomena, spatial numerical resolution is a crucial ingredient,therefore requiring the use of an adaptive mesh refinement (AMR). These previous works assumed that the solar rotation and the magnetic axis were aligned. Here we present including, for the first time, the tilt of the heliocurrent sheet using a 3D MHD AMR simulation with BATS-R-US code. We discuss the effects on the global structure of the Heliosheath, the flows, turbulence and magnetic field structure. We access the consequences for the observations measured by Voyager 1 since mid-2002. This intensive computational run was done at the supercomputer Columbia at NASA/AMES.



April 20, 2005
Dr. John J. Vajo
Hughes Research Laboratories, LLC

Title: Hydrogen Storage Materials for Transportation Applications

A commercially viable technology for hydrogen storage is a major requirement for widespread use of fuel cells in transportation applications. This presentation will review various hydrogen storage technology options. Broadly, hydrogen can be physically contained as gas or liquid, or chemically stored through interaction with a storage material. Chemical storage materials can be classified into reversible materials, which may be recharged directly with hydrogen, and irreversible materials, which require more extensive chemical processing. Further, directly rechargeable materials can store hydrogen molecularly, i.e., as a H2 molecule, or dissociatively as chemically bound hydrogen atoms. Examples of each of these types of materials will be discussed with an emphasis on systems studied at HRL. In particular, we will describe an approach for thermodynamically destabilizing low atomic number (low-Z) hydrides. These materials have favorably high hydrogen contents but typically are chemically very stable. This stability makes hydrogen release and direct recharging impractical. Destabilization can potentially yield materials that reversibly store large amounts of hydrogen at moderate conditions.



April 27, 2005
Dr. Guanghong Wei
Department of Chemistry and Biochemistry
University of California Santa Barbara

Title: Conformational sampling of A? (25-35) peptide in aqueous solution and membrane-mimicking environment using molecular dynamics simulations

Interest in the beta amyloid (AΒ) peptides continues to grow due to their known accumulation in the form of insoluble fibrillar aggregates, in the brains of patients with Alzheimer's disease. The aggregation process from the soluble peptide to Alzheimer's disease amyloid plaque is believed to involve a conformational switching from helix/random coil to a Β-sheet structure. Hence, the conformations of monomeric AΒ in membrane-mimicking environment and in water, are of significant interest. AΒ (25-35), sequence GSNKGAIIGLM, is a biologically active fragment of AΒ. In this study, the conformational space of this peptide in pure water and in 80% (in volume) Hexafluoro-prooanol (HFIP) solution (membrane-mimicking environment) were explored using four independent replica exchange molecular dynamics (REMD) simulations for a total of 2.46 µs. The structural dynamics of AΒ (25-35) in water and 80/20 (v/v) HFIP/water are studied using contanst temperature molecular dynamics (CTMD). Our simulations show that A adopts Β-hairpin structure in water and helical structure in HFIP/water mixture. In the Β-hairpin structures, the Β-turn is exposed to water and the side chains of the amino acid residues in the adjacent positions in the sequence are oriented on opposite faces of the sheet, which might promote the aggregation of Β-sheet. The role of the solvent environment in the different conformational preference are discussed.



May 4, 2005
Dr. Frances Hellman
Department of Physics
University of California, Berkeley

Title: Spin Electronics: Magnetic Moments and Amorphous Semiconductors

Spin electronics in its broadest definition is the study of systems where both the charge and the spin of the electron play a role. The charge of the electron usually is important because we measure currents and voltages. The spin of the electron is most obviously seen only in magnetic materials. Examples of spin electronics range from technological ideas such as MRAM (magnetic random access memory) which are based on magnetic tunnel junctions, to some forms of quantum computing. I will discuss efforts to introduce magnetic moments into semiconducting materials, including our work on amorphous Si doped with magnetic ions such as Gd. These alloys possess dramatic magnetic and transport properties due to electron-electron and electron-local moment interactions, including enormous (many orders of magnitude) negative magnetoresistance.



May 18, 2005
Dr. Johnny Powell
Department of Physics
Reed College

Title: Phonons and the Core Complex

Condensed matter theory and experiment have provided explanations for the "melting" of DNA, here it will be argued that similar methods could be used to explain the melting of the core complex. The core complex is a tightly bound group of highly symmetrically interacting alpha helices that is essential for synaptic transmission between neurons. The core complex must disassemble for effective transmission of information. A detailed case will be made that the self-consistent phonon approximation (SCPA) could be applied to the core complex to explain-on an atomic level-disassembly of this crucial molecular complex. An experiment using evanescent wave microscopy and fluorescence resonance energy transfer will be proposed to test the putative predictions of the SCPA.