Physics Colloquium

Spring 2009

Thursdays 4:00 p.m., Room 104 Physics
Refreshments served at 3:40 p.m.
  Colloquium organizer: Alexey Yamilov
(Link to main colloquium page)

Green - open date
Blue - tentative (reserved)
Red - firm commitment

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Date Host Speaker Title of the talk Abstract
Jan. 15 Yamilov Andrea Markelz,
SUNY Baffalo
Correlated motion and protein dynamics:  what terahertz spectroscopy tells us
Proteins are large macromolecules that perform biological processes through large scale structural changes.  The underlying mechanisms of protein function are still uncertain. Does correlated motion cause large scale structural motion, or does diffusive small scale motions move the system towards its final lowest energy conformation? Normal mode calculations of proteins determine the lowest frequency structural vibrational modes are in the terahertz frequency range and measurements have demonstrated a correlation between the terahertz dielectric response and protein function. But these measurements alone do not demonstrate correlated structural motion.  Here we discuss experimental THz dielectric measurements of the hydration dependence of cytochrome c and if this hydration dependence can be fully explained by structural motions alone, or if in fact diffusive motions dominate.
Jan. 22 Hale Timothy Gay,
University of Nebraska Lincoln
Football Physics This talk discusses a series of one-minute physics lectures given to the ~ 8 x 10^4 fans that attend the University of Nebraska home football games.  The lecture topics range from gyroscopic motion to ionizing collisions between linebackers and I-backs. The problem of simultaneous edification and amusement of the fan in the stands is considered. 
Jan. 29 Madison  James Colgan,
Los Alamos National Lab 
Atomic physics using high-performance computing
The last 25 years or so have seen a spectacular increase in raw computing power, both in processor speed and memory resources, as well as the availability of supercomputing facilities. This increase has allowed larger calculations and numerical simulations to be performed in all areas of physics, greatly enhancing understanding of many physical processes.

This computing power increase has also significantly impacted atomic collision physics. This talk will describe how large-scale computing has facilitated investigations of fundamental problems in collision physics, such as the so-called Coulomb three-body problem, which has no analytic solution and has, until recently, defied numerical solution. Examples will be drawn from several recent areas in which progress has been made. Some other areas in which high-performance computing may be expected to impact on atomic collision physics will also be described.
Feb. 5  
     
Feb. 12  
     
Feb.  19 Yamilov
Carl M. Bender,
WashU
Making Sense of Non-Hermitian Hamiltonians The average quantum physicist on the street believes that a quantum-mechanical Hamiltonian must be Dirac Hermitian (symmetric under combined matrix transposition and complex conjugation) in order to be sure that the energy eigenvalues are real and that time evolution is unitary. However, the Hamiltonian $H=p^2+ix^3$, for example, which is clearly not Dirac Hermitian, has a real positive discrete spectrum and generates unitary time evolution, and thus it defines a perfectly acceptable quantum mechanics. Evidently, the axiom of Dirac Hermiticity is too restrictive. While the Hamiltonian $H=p^2+ix^3$ is not Dirac Hermitian, it is PT symmetric; that is, it is symmetric under combined
space reflection P and time reversal T. In general, if a Hamiltonian $H$ is not Dirac Hermitian but exhibits an unbroken PT symmetry, there is a procedure for determining the adjoint operation under which $H$ is Hermitian. (It is wrong to assume a priori that the adjoint operation that interchanges bra vectors and ket vectors in the Hilbert space of states is the Dirac adjoint. This would be like assuming a priori what the metric $g^{\mu\nu}$ in curved space is before solving Einstein's equations.)
Feb. 26  
     
Mar. 5 Medvedeva Mariana Bertoni,
MIT
Defect Engineering of Solar Cell Materials    Multicrystalline silicon (mc-Si), which currently accounts for almost 50% of worldwide solar cell module production, offers a promising alternative energy source mainly due to its low production cost and scalability. However, in order to become a grid-competitive energy alternative, higher efficiencies should be obtained. The major disadvantage of mc-Si is the inhomogeneously distributed regions of low minority carrier lifetime, which drastically reduce the efficiency of the overall cell. These highly defective regions are a consequence of the presence of metallic impurities and/or dislocation lines.
   To close the efficiency gap between industrial multicrystalline and high-efficiency monocrystalline silicon solar cells it is crucial to understand the structural defects present and the role they play in the silicon matrix. To improve the performance of mc-Si based solar cells, a manufacturable method to mitigate the impact of dislocations on the electrical properties and to engineer the distributions of metal-impurity nanodefects in a controlled fashion must be developed.
   Defect-engineering approaches can achieve homogeneous and high bulk minority carrier lifetimes (in the 100 �s range) in low-cost crystalline silicon substrates, in combination with defect-tolerant cell processing technologies, resulting in high (≥18-22%) efficiency solar cells at low cost (≤$1.00/Wp). The technologies proposed herein can be implemented on an industrially meaningful scale, supporting factories up to and potentially beyond the 1 GWp/yr range.
Mar. 19 Yamilov  Amitava Choudhury,
Missouri S&T (Chemistry)
Many Virtues of Transition Metal Phosphates: Magnetic Porous Solids to Li-ion Batteries    In the materials world transition metal phosphates belong to a class of their own. The unique chemistry of transition metals when coupled with the tetrahedral phosphate (PO43-) building block produces architectures which can have a vast range of applicability from magnetic porous solids to Li-ion batteries. Porous inorganic materials constitute an important area of research in materials chemistry, because of their potential applications in areas such as separation, sorption, and catalysis. Porous materials are best exemplified by a class of compounds known as zeolites which are constructed by the corner-sharing of the AlO4 and SiO4 tetrahedra. Besides aluminosilicate zeolites, the metal phosphates constitute a large family of porous structures. These metal phosphates exhibit fascinating architectures with unusual bonding and coordination patterns not seen in aluminosilicate zeolites. More importantly, phosphate-based frameworks when synthesized with appropriate transition metal can exhibit interesting magnetic property leading to ferromagnetic porous solids. Synthesis, structure and some important properties of these materials would be discussed. Recently, another virtue of transition metal phosphates has been unveiled as the safe, environmentally benign and cheap choice of cathode material for Li-ion batteries. For example, olivine structure of LiFePO4 has been touted as the cathode material for Li-ion battery that will be used in the next generation hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV) and battery electric vehicle (BEV). Some of the basic features of these materials will be discussed and some results of the studies directed towards the search for new phosphate based materials for Li-ion battery will be presented.
Apr. 2 Yamilov Jonathan Katz,
WashU
Mpemba Effect: When Hot Water Freezes Before Cold I suggest that the origin of the Mpemba effect (the freezing of hot water before cold) is freezingpoint depression by solutes, either gaseous or solid, whose solubility decreases with increasing temperature so that they are removed when water is heated. They are concentrated ahead of the freezing front by zone refining in water that has not been heated, reduce the temperature of the freezing front, and thereby reduce the temperature gradient and heat flux, slowing the progress of the freezing front. I present a simple calculation of this effect, and suggest experiments to test this
hypothesis.
Apr. 9 Dubois John Tanis,
Western
Michigan U.
Fast Electron and Ion Transmission through Insulating Nano- and Microcapillaries

   Characteristics and interactions of electrons and ions traversing nano- and micrometer sized electrically insulating capillaries are investigated. Arrays of tightly packed capillaries in thin foils, as well as single cylindrically shaped and tapered glass capillaries are studied. The investigations represent an emerging interdisciplinary area of nanoscience lying at the intersection of atomic physics and materials science, posing interesting questions of a fundamental nature, as well as offering potential applications in science, medicine, and technology. The studies focus on two distinct aspects of charged particle transmission in the capillaries: (1) the underlying mechanisms of charged particle interactions with insulators, and (2) the production of micron- and submicron-sized charged particle beams and their properties. In the former case, the goal is to identify and understand the fundamental interactions that occur, including both elastic and inelastic scattering processes, when charged particles traverse highly-ordered structures of nano- and micrometer size. In the latter case, different geometrical configurations and different types of insulating capillaries are used to examine the characteristics of ions and electrons transmitted through the capillaries.

Apr. 16 Yamilov Ray Arvidson,
WashU
The Phoenix Lander Mission:
A Trip to the Frozen Arctic of Mars

The Phoenix Lander touched down at ~68 degrees north latitude on plains dominated by cryoturbation landforms in May 2008 and conducted operations for 152 sols. Thirty one samples (soils and icy soils) were delivered by the 2.4 m long robotic arm to instruments on the Lander deck. A dozen trenches were excavated into the top and sides of polygons and within the troughs between polygons. Water ice deposits were found several centimeters beneath wind-blown soil materials. Soils are surprisingly dry, dominated by basaltic components, and slightly cohesive. We will explore whether or not the soils have been pedogenically processed during periods of high obliquity, when warm, relatively moist conditions may have allowed migration of thin films of water to and from the ice table.

Apr. 23 Jentschura Peter J. Mohr,
NIST
The status and future of fundamental constants A new set of values of the fundamental constants of physics and chemistry has been recommended by the CODATA Task Group on Fundamental Constants. These values are based on the data available up to the end of 2006 and are called the 2006 recommended values.  They are based on the definitions of the International System of Units (SI).  It is expected that some of the definitions of SI units will change in the near future, and this will have a significant impact on the values of the fundamental constants.  Among other things, the redefinition will result in many of the constants having exact values, whereas they currently have uncertainties that are associated with the unit definitions.  The changes in the SI system that are expected and the effect on the values of the fundamental constants for the future will be reviewed 
Apr. 30 Yamilov Vasily Astratov,
U. of North Carolina - Charlotte
Optical Transport Phenomena in Coupled Spherical Cavities This talk is devoted to “mesophotonics” – novel area of photonics dealing with the optical properties of coupled cavities with dimensions in the order of several wavelengths. In such structures the light can be trapped in cavities at the resonant frequencies, and it can tunnel from cavity to cavity under resonant conditions. Some insight into the optical transport properties can be obtained due to a tight binding approximation for photonic atoms. However the behavior of real physical coupled cavity systems is complicated by to the disorder-induced light localization and scattering effects. The focus of this presentation is on developing new concepts related to light transport in systems of coupled microspheres including photonic nanojet-induced modes, percolation of whispering gallery modes and the role of structural disorder and dimensionality. At the device level, these studies stimulate developing designs where coupled cavities are applied to tight focusing micro-probes, sensors, and compact spectrometers.
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