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Physics and Astronomy Seminars: Fall 2006 Schedule

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Department of Physics and Astronomy
George Mason University, Fairfax

Time:	Fridays, 11 AM - noon
Place:	Room 301, Research I

Driving Directions to GMU

Visitor parking decks

September 1: Bob Ehrlich, Chair of Physics and Astronomy, George Mason University, Fairfax, VA

Crazy Ideas in Science

Abstract:

In this talk I consider how crazy ideas can sometimes lead to great scientific advances, and even cause us to re-examine the nature of science itself.  This light-hearted talk will also serve as a template for some creative and humorous uses of Powerpoint that can add interest to your own presentations.

September 15: James P. Yesinowski, Materials Chemistry Branch, Naval Research Laboratory, Washington DC

^9,71Ga and ¹⁴N NMR of Gallium Nitride as a Probe of Structure & Electronic Charge/Spin Effects of Dopants and Defects
Abstract:

Gallium nitride, a wide bandgap III-V semiconductor, is the subject of intensive R&D worldwide because of its many electronic and optoelectronic applications. In this talk I will illustrate how NMR studies of the quadrupolar nuclei in GaN can provide unique information on structure and the effects of electronic charge and spin resulting from dopants and defects.

The NMR parameters that have been used to obtain such information are chemical shifts and Knight shifts, Nuclear Quadrupole Coupling Constants (NQCC), and T₁ and T₂ relaxation times. Chemically-synthesized polycrystalline samples have been studied primarily by MAS-NMR, and single-crystal film samples by static NMR, which enables measurement of anisotropic parameters. Comparisons of ⁶⁹Ga and ⁷¹Ga (both I=3/2) T₁ values distinguish between magnetic and quadrupolar relaxation mechanisms; an approach to sorting out the complicated relaxation behavior of the four-level I=3/2 spin system will be briefly discussed.

Dramatic changes in the ⁷¹Ga and ¹⁴N (I=1) MAS-NMR spectra for chemically rather minor level of dopants (0.13% Ge) will be shown to arise from Knight shifts due to degenerate conduction electrons, leading to Korringa-type T₁ relaxation. I will show how the inhomogeneous character of the distribution of Knight shifts can be utilized to obtain the probability density function of the (electron) carrier concentration, a distribution due to spatially-varying dopant concentrations.

Other aspects to be discussed include cubic and hexagonal GaN polytypes, distributions of electric field gradients at the Ga and N sites arising from defects/dopants, and novel dynamic equilibrium effects arising from defects/dopants. In addition, a variety of approaches to significantly enhance the detection sensitivity of ⁷¹Ga NMR of films will be mentioned.

September 29: Menas Kafatos, Co-Dean of College of Science, George Mason University, Fairfax, VA

Arrow of Time, Complementarity and Cosmology

Abstract:
Theories of fundamental physics and cosmology must ultimately not only account for the structure and evolution of the universe and fundamental interactions, but also lead to an understanding of why this particular universe follows the physics that it does, including the values of the fundamental constants. The usual approach is to assume the constants are invariant and time evolves. Here we explore a different paradigm where the constants themselves are varying. This gives rise to an apparent evolution, equivalent to an apparent expansion of the universe. Starting from a set of relationships proposed by Weinberg, we show that this approach is equivalent to the Dirac/Eddington ratios of fundamental constants or parameters. A different concept of time and a view which is complementary to the usual evolution of the universe arise. We discuss these concepts in the framework of a generalized foundational principles, including a generalization of Bohr's complementarity.

October 13: Nathalia Peixoto, Professor, Department of Electrical and Computer Engineering, George Mason University, Fairfax, VA

Bioengineered Sensors and MEMS Devices for Neural Research

Abstract:
Microfabricated structures are critical tools for the investigation of biological systems. In many cases these devices form the interface which allows engineering techniques and methodologies to be applied.  Two classes of devices will be explored in this presentation: microfabricated sensor arrays and electrodes.  Initially I will discuss my experimental results obtained using microelectrodes on spreading depression waves, a well-established model for migraine visual effects, and present some cellular automata based modeling of the observed spiral waves.  Next, I will focus on applications of microelectrode sensor arrays and highlight experiments with small biological neural networks that grow on the array structure itself. Using clustering analysis techniques, I will show the interaction of electrical stimuli with these neural networks.  Finally, I intend to describe the design and use of implantable, biocompatible electrodes for seizure control in animals.

October 27: David Geho, Research Professor, Center for Applied Proteomics &. Molecular Medicine, George Mason University, Prince Williams, VA

The role of clinical proteomics in personalized medicine

Two hallmarks of personalized medicine are early detection of disease processes and patient stratification based on molecular analyses of diseased tissues. Both of these endpoints require the development of new clinical test systems based on interdisciplinary approaches. The purpose of this lecture is to describe the goals of personalized medicine and to describe areas of potential collaboration with disciplines outside of medical practice.

November 3: Mike Summers, Professor, Department of Physics and Astronomy, George Mason University, Fairfax, VA

Ares: The Mars Airplane Mission

ARES (Atmospheric Regional-scale Environmental Survey of Mars) is envisioned to be the first airplane to fly on another planet. As such, it is the most ambitious and technically challenging robotic planetary mission ever designed. But the scientific return of this mission could be enormous. The ARES airplane will include a suite of instruments with primary goals to (1) explore the complex planetary magnetic field and near-surface radiation environment, (2) make the first in-situ maps of the atmosphere’s chemical and dynamical structure, and (3) search for metabolic by-products (biomarkers) of putative subsurface life. The ARES observations will thus have special significance for understanding the geological, and possible biological, evolution of the planet. ARES will also scout for potential landing sites for future human exploration. In this talk I will review some of the scientific issues relating to Mars and give an overview of the ARES mission.

November 17: Victor Debattista, Brooks Fellow, Astronomy Department, University of Washington, Seattle, WA
Secular Evolution of Galaxies: From Dark Halos to Nuclei

One path by which disk galaxies evolve is via internal secular evolution.  By breaking the symmetry of the system, bars play an important role in driving such evolution.  I describe some recent results concerning aspects of this evolution relating to dark halos, disk density profiles, bulges and central nuclei.

December 8: Vladimir Ivanov, Postdoctoral Fellow, Department of Physics and Astronomy, George Mason University, Fairfax, VA 
Atomistic studies of grain boundary motion in metals

Properties of metallic materials are strongly influenced by the presence of internal interfaces. Grain boundaries, i.e. interfaces between differently oriented crystallites of the same phase (grains), play a significant role in many properties of materials such as strength, fracture, plasticity and corrosion resistance and many others.

An important feature of grain boundaries is the strong dependence of their properties on crystallographic characteristics. That is why characteristics of grain boundaries are so important for understanding microstructure evolution of materials.

Computer modeling of grain boundary migration (GBM) involves large ensembles of atoms and requires statistical averaging over many atomic events. It is made possible by using semi-imperial interatomic potentials, which allow fast calculations of the total energy and interatomic forces. Molecular Dynamics (MD) simulations are often used to study GBM as they are in many respects very similar to real experiments. Such simulations give fundamental insights into atomic mechanisms and dynamics of grain boundary migration.

In this talk, MD and other methods of grain boundary modeling are reviewed. Recent studies of atomistic modeling of concurrent GBM and shear deformation in aluminum are presented. The results are summarized as a general formulation of plasticity resulting from grain boundary motion coupled to shear deformation.

Questions about the physics seminar? Contact Karen Sauer at ksauer@physics.gmu.edu

Other interesting seminar series at GMU:

College of Science Seminars

Seminar in Bioinformatics and Computational Biology

Chemistry and Biochemistry Seminar

COLLOQUIA of the Computational Materials Science Center

Krasnow Monday seminar series

Quantum Information Science (QIS) Seminar

Math Colloquium

Physics Seminar Schedule for previous year: Fall2005, Spring2006