Fall 2003

December 10, 2003
Dr. Tom Berger
Lockheed-Martin Astrophysics Laboratory
Palo Alto, California

Title: Imaging the Solar Photosphere with 0.1-arcsecond Resolution

Abstract: Recent data from the Swedish 1-meter Solar Telescope (SST) on La Palma have revealed new structures and dynamics in sunspots, active region convection, and magnetic elements in quiet Sun and near the solar limb. The SST is a vacuum refractor incorporating a 37-element adaptive optics system and real-time frame selection to achieve 0.1-arcsecond resolution of the solar photosphere for the first time. Initial results from the first summer of observing in 2002 show that bright penumbral filaments have dark central cores approximately 100 km in diameter, "canals" of highly magnetically constrained convective downflows exist near pores and sunspots, granulation at the edges of pores appear fluted in a "hair"- like, possibly proto-penumbral, manner, and that sunspot lightbridges can exhibit a dark central lane and convective overturning motions and flows on 100-300 km scales. Recent magnetogram observations of the quiet Sun taken with the Lockheed Solar Optical Universal Polarimeter (SOUP) birefringent filter and simultaneous G-band 430 nm, continuum, and Ca II H- line filtergrams reveal strange magnetic element structures that have yet to named or understood. Taken as a whole, the SST results approach the spatial resolution of the best contemporary magnetohydrodynamic (MHD) models and pose significant challenges to these same models to explain the new structures, none of which has been previously observed or modelled.



December 3, 2003
Prof. William Klug
Department of Physics
UCLA

Title: A Director-Field Theory of DNA Packaging in Bacteriophage Viruses

Abstract: The packaging of DNA into a typical bacteriophage virus can be thought of as geometrically analogous to fitting 400 feet of electrical extension cord into a basketball. Nature's performance of such a remarkable feat is impressive, and we can view it as an interesting challenge to analyze and understand this geometrically and mechanically rich process. I will present a continuum theory of viral DNA packaging based on a director-field representation of the packaged DNA. Packaged conformations are identified as minimizers of a total free energy functional, which is composed of mechanical and electrostatic contributions. I will discuss the usefulness of this approach for gaining insight into established packaging models and for suggesting new packing motifs, and compare analytical predictions of the theory to experimental observations. I will also present an adaptation of the theory to a framework of numerical optimization, wherein all fields are discretized on a computational lattice, and energy minimizing configurations are sought via simulated annealing and the nonlinear conjugate gradient method.



November 12, 2003
Prof. Donald Jacobs
Physics and Astronomy
Cal. State University Northridge

Title: "Predicting Protein Stability form a Free Energy Decomposition: A Physicist's Quest for Missing Entropy"

Abstract: Lord Kelvin said, "I never satisfy myself until I can make a mechanical model of a thing. If I can make a mechanical model I can understand it!" Adhering to Kelvin's belief, a mechanical model to understand protein flexibility and thermodynamic stability is presented. Many salient features of thermodynamic stability found in polypeptides and proteins are well described using a novel Distance Constraint Model (DCM). Microscopic interactions such as covalent bonds, salt bridges, hydrogen bonds and torsion-forces are modeled as constraints. A topological arrangement of constraints defines a mechanical framework. Each constraint in the framework is associated with an enthalpic and entropic contribution. Total enthalpy is obtained by simple addition over all enthalpic contributions within a framework. The non-additivity of entropic components is calculated using network rigidity. Instead, total entropy is additive over a preferential set of independent constraints, accounting for missing entropy. Network rigidity is a mechanical interaction that provides a mechanism for molecular cooperativity. The DCM is applied to polypeptides that undergo the alpha-helix transition, where the partition function is calculated exactly using a transfer matrix approach. DCM predictions are compared with Monte Carlo simulation data for polyalanine in aqueous solution. Under suitable mixed solvent conditions heat and cold denaturation is predicted, in excellent agreement with experiment. In addition, a free energy landscape is calculated for Ubiquitin within aLandau-like mean field description. Heat capacity and stability curves are compared with experiment.



November 5, 2003
Prof. Tommaso Roscilde
Physics and Astronomy
University of Southern California

Title: "Quantum affects on the collinear phase of the frustrated J1-J2 antiferromgnet"

Abstract: The frustrating J1-J1 antiferromagnet on the square lattice is a matter of intense debate. As for its ground state properties, large evidence is provided for a quantum non-magnetic ground state for strong frustration, but its nature and stability is yet vaguely understood. On the finite-temperature side, the spontaneous breaking of a discrete lattice symmetry in the collinear phase, or Chandra-Coleman-Larkin (CCL) transition, predicted in the classical limit, has recently challenged by analytical and numerical results in the case S=1/2. In this work we make use of a semi classical approach called PQSCHA (pure-quantum self-consistent harmonic approximation) to study the collinear phase of the model for arbitrary spin. At zero temperature we determine the renormalized spectrum of excitations and the critical value of frustration where collinear order disappears. At finite T, we calculate the renormalized critical temperature of the CCL transition for arbitrary spin value, observing that the transition persists down to the extreme quantum limit S=1/2. Our results for the thermodynamics are discussed in connection with recent experiments on the vanadium oxide Li2VOSiO4, realizing one of the first prototypes of the quasi-2D J1-J2 antiferromagnet with large J2.



October 22, 2003
Prof. Camelia Prodan
Physics
University Cal. Santa Barbara

Title: "On The Dielectric Properties Of Live Cell Suspensions"

Abstract: The dielectric properties of live cell suspensions are theoretically and experimentally investigated in the limit of low frequencies and applied electric fields. The live cell suspensions are modeled as suspensions of arbitrarily shaped, shelled and charged particles. The theoretical results are valid in the low range of frequencies (alpha dispersion) as well as in the high range of frequencies (beta dispersion). Our numerical simulations reveal a very strong dependence of the dispersion curves on the membrane potential. The dielectric response of live cell suspensions is then measured and fitted with the theoretical model. A value of the membrane potential, which is in agreement with previously reported values, is obtained.



October 15, 2003
Prof. Jianping Hu
Physics and Astronomy
University Cal. Los Angeles

Title: "High dimensional generalization of Quantum Hall Effect"

Abstract: Quantum Hall Effect has been on of the most important and active research fields since it was discovered in a two-dimensional electron gas system in 1981. It is an great example to learn how to organization principles can be emerged in condensed matter systems. Moreover, it is deeply connected to the fundamental mathematical structure. In this talk, well will show the QHE can be generalized to high dimensional spaces. The new physics in the high dimensional quantum Hall systems may provide us an approach to unification of relatively and quantum mechanics.



October 8, 2003
Prof. Andrea J. Liu
Chemistry and Biochemistry
University Cal. Los Angeles

Title: "Jamming"

Abstract: Jamming occurs when a system develops a yield stress or extremely long stress relaxation time in a disordered state. According to this definition, many systems jam. Granular materials, foams or emulsions can flow when they are sheared, but jam when the shear stress is lowered. These systems are athermal; random motions supplied by some driving force are necessary to induce any motion since thermal energy is insufficient to cause particle rearrangements. Thermal systems can also jam; colloidal suspensions of small particles jam as the packing density is raised and supercooled molecular and polymeric liquids jam as the temperature is lowered-this is the glass transition. We have proposed that the temperature, stress and packing fraction are important parameters that control jamming for all systems, and that the state of the system can be represented by a "jamming phase diagram." I will discuss numerical simulations on model glassforming systems that explore such a diagram, with particular focus on the onset of jamming with increasing packing density at zero temperature and shear stress.



October 3, 2003
Prof. Jeffrey L. Vaughn
Optical Sciences Company

Title: " Adaptive Optics Physics Done with Smoke and Mirrors"

Abstract: Atmospheric turbulence or the "mirage effect" blur and distorts images form optical systems. The ability to correct or compensate for atmospheric turbulence allows optical systems to work at full resolution. For example, the 200" telescope on Mount Palomar has no better resolution than an 8" high quality telescope that a serious amateur astronomer might own. With adaptive optics, the resolution can be increased by 25 times or nearly 3 times the resolution of the Hubble Space Telescope. This technology is currently being used to improve the resolution of ground-based telescopes and is part of the beam control of the Air Force's AirBorne Laser. Future applications include free-space laser communications, imaging through refractive and turbulent media, control of chemical and nuclear reactions, and laser surgery.



September 24, 2003
Prof. Sargis Dallakyan
Physics and Astronomy
Cal State University Northridge

Title: "Shot Noise in Molecular Wires"

Abstract: With constantly decreasing size of electronic devices, we are approaching a limit where single molecules can act as rectifiers, diodes or even logic circuits. In the present talk, I will briefly review recent theoretical and experimental work related to molecular wires. After presenting the formulas for the current and noise power based on scattering matrices, I'll discuss on how to use nonequilibrium Green's Function (NEGF) formalism in odder to calculate transmission probability through molecular wire. As an example, we calculate current and noise power for polyenes molecules, and show that hey decrease with increasing size of the system. Even with symmetric connection to metallic contracts, current-versus-voltage curves can be asymmetric for asymmetrically substituted polyenes. Most importantly, we demonstrate a cross-over form Poissonian to sub-Poissonian behavior in the shot noise as a function of applied voltage. The algorithm for noise power calculation can be used for deigning molecules with low noise.



September 17, 2003
Prof. Jose Rodriguez
Physics and Astronomy
Cal State University Los Angeles

Title: "Defective Vortex Lattices in High-Temperature Superconductors"

Abstract: High-temperature superconductors are layered materials and extremely type II. This makes the Abrikosov vortex lattice state that arises from the application of external magnetic field (oriented perpendicular to the layers) very sensitive to thermal fluctuations and to quenched-in material defects. We study theoretically the nature of the defective vortex lattices that result by employing duality techniques known from statistical mechanics. Certain features of high-Tc phase diagram, like the recently observed vortex-liquid/vortex-liquid transition, can be accounted for by this approach.