Fall 2004 Overview

Date	Speaker	Institution	Topic
Sep 22 2004	Valentina Abramenko	BBSO/NJIT	solar flares, magnetic fields, fractals
Sep 29 2004	Vittorio Cristini	UCI	simulations biophysics
Oct 06 2004	External Review
Oct 13 2004	Dmitri Semikoz	UCLA	Neutrinos in the early universe
Oct 20 2004
Oct 27 2004	Debi Prasaad	CSUN	solar flares and magnetic fields
Nov 03 2004	Daniel Cox		Electronic properties of DNA
Nov 10 2004	Merav Opher	JPL	plasma/interplanetary
Nov 16 2004	Nick Kioussis	CSUN - Bianchi Seminar	Computational Materials Theory - What can we learn?
Nov 17 2004	No Seminar
Nov 19 2004	Dave Aspnes	North Carolina State University	The Microscopic Origin of Optical Properties
Nov 24 2004	Thanksgiving Talk?
Dec 01 2004	Michael Dennin	UCI	NLD, sheared foam, domain growth

Fall 2004 Abstracts

November 19, 2004
Dr. David E. Aspnes
Department of Physics, North Carolina State University

Title: The Microscopic Origins of Optical Properties and the Simplified Bond Hyperpolarizability Model of 2nd-, 3rd-, and 4th-Harmonic Generation

The idea that reflection of light from a material can be described at the atomic level as a coherent superposition of radiation of dipoles driven by incoming radiation was developed for linear optics by Ewald in 1912 and independently by Oseen in 1915, and is summarized in the largely forgotten Ewald-Oseen extinction theorem. We recently applied the same concepts to second- (SHG)1 , third- (THG)2, and fourth-harmonic generation (FHG)3 from silicon, showing that this simple approach not only provides a new understanding of nonlinear-optical processes in general but also greatly simplifies the analysis of SHG and FHG data of Si-dielectric interfaces and THG data from bulk Si. Here, I will review the Ewald-Oseen approach, then discuss generalizations to SHG, THG, and FHG leading to the simplified bond-hyperpolarizability model (SBHM)of nonlinear optics (NLO). In addition to reducing the number of parameters describing SHG data for pp, ps, sp, and ss polarizations to 3 from the 11 and 14 needed for the Fourier and tensorial representations, respectively, I show that the SBHM allows the presence of complex hyperpolarizabilities to be readily recognized from SHG anisotropy data. For FHG the presence of intrinsic and interface-roughness contributions is identified. The THG case is particularly interesting because the SBHM predicts observed lineshapes with no adjustable parameters. The capability of such a simple model to describe a wide range of NLO phenomena in Si suggests that the primary NLO contributions are geometric, and as a result that the amount of interface information available from single-wavelength measurements is limited. The obvious next step is spectroscopy.

1. G. D. Powell, J.-F. T. Wang, and D. E. Aspnes, Phys. Rev. B65, art. no. 205320 (2002); J.-F. T. Wang, G. D. Powell, R. S. Johnson, G. Lucovsky, and D. E. Aspnes, J. Vac. Sci. Technol. B2, 1699 (2002).
2. H. J. Peng and D. E. Aspnes, Phys. Rev. (in press).
3. J.-K. Hansen, H. J. Peng, and D. E. Aspnes, J. Vac. Sci. Technol. B21, 1793 (2003).



November 16, 2004
Dr. Nick Kioussis, recipient of the 2003-04 Donald E. Bianchi Faculty Award Department of Physics and Astronomy, California State University, Northridge Bianchi Seminar given at the Donald E. Bianchi Planetarium at 3:00 pm.

Title: Computational Materials Theory - What can we learn?

The past decade has seen a rapid rise in the use of computer simulations in material research. Due to phenomenal advances in computer power and accessibility, simulations now guide discovery and understanding of materials behavior in nanoscience and technology. I will give an overview of the research work we have carried out in the past several years in a wide range of areas: From applying first-principles methods based on Density Functional Theory, to specialized models for highly correlated electron nanosystems, to multiscale approaches that seek to link different length scales sequentially to understand mechanical properties, and to methods for studying electron transport in magnetic nanodevices.



November 3, 2004
Dr. Daniel L. Cox
Department of Physics, University of California, Davis
and Center for Theoretical Biological Physics, UCSD

Title: Electronic Properties of DNA (?)

In the past few years, with new tools from nanoscale science and high speed optics, considerable attention has been focused on an old question: can DNA conduct? Single molecule and optics experiments in the physics community have yielded answers ranging from complete insulator through ohmic conductor at room temperature to induced superconductivity (!), while a variety of experiments from the chemistry community have shown the possibility of relatively fast charge transfer compared to other biomolecules and intriguing long ranged electron damage and healing processes in the lab, leading to fascinating speculations about electron assisted searching for damage in DNA. I will review and critique existing data and present theoretical evidence from new molecular dynamics/electronic structure calculations for hydrated DNA in the presence of positive counterions revealing surprising possibilities for low activation energy but low mobility conduction.



October 27, 2004
Dr. Debi Prasad Choudhary
Department of Physics and Astronomy, California State University, Northridge

Title: Halloween Storms on the Sun in 2003

During September 2 to October 18 2003, around Haloween, the Sun became suddenly violent with the appearance of large sunspot complexes. The storms thatoccurred at the spot complexes were the most powerful since recorded history. In this talk we present the results of our investigation dealing with the triggering mechanism of these flares. We used ground based observations along with space based data to study the magnetic complexity of the groups of sunspots where these flares occurred. We find that the localized reconnections of magnetic field led to the powerful flare events. We show that this could be a common underlying mechanism for the triggering of large flares on the Sun.



September 22, 2004
Dr. Valentina Abramenko
Big Bear Solar Observatory and New Jersey Institute of Technology

Title: Intermittency of the Magnetic Fields on the Sun

Universality of nonlinear dissipation processes in nature (from sand and snow avalanches, forest fires, earthquakes to nuclear chain reactions, cosmic rays, magnetospheric substorms, solar flares, etc.) inspires our efforts to explore them. Such processes can be described as an evolution of a nonlinear dissipative dynamical system. The ubiquitous characteristic of such a system is that an input of energy is accompanied by a nonlinear dissipation process, so that the system evolves to chaos, when unpredictable (in space and in time) bursts of energy dissipation on all scales may occur. Spatial and temporal structures of such a system display intermittency, i.e. the tendency to concentrate in strong small-scale features separated by extended areas of lowfluctuations.

Magnetized plasma in the solar atmosphere is a typical example of such a system. The Sun provides us with an opportunity to study magnetized plasma, which is not available in a ground-based experiment: due to huge spatial scales (as compared to laboratory spatial scales) the magnetic field is frozen into solar plasma. Therefore, the solar magnetic field subjects to chaotic plasma motions. This effect results in creating of numerous fragmented magnetic flux tubes. Interactions between these flux tubes lead to nonlinear energy dissipation on a wide range of scales: from small-scale energy release events (nanoflares) to major solar flares accompanied by coronal mass ejections, which may produce a noticeable impact on the Earth's magmetosphere.

We focus on a study of the magnetic field structures on the surface of the Sun, on an analysis of their intermittency. Our approach is based on a realistic assumption: the photospheric plasma embedded in a magnetic field is in a turbulent state. The method allows us to determine the role of intermittency of the magnetic field in the evolution of an active region as a whole, and in its flaring activity, in particular.