Title

“3-Cycle and Chaos in the Logistics Map and Sharkovskii's Theorem”

As a prototypical 1d chaotic map, the logistic map has played an
important role in elucidating the chaotic behavior in nature. By
necessity nearly all the studies on this map have been numerical and
numerically driven. It has been found recently that the dynamical
behavior of 3-cycle in this map can actually be studied analytically.*
There is a new theorem by a Ukrainian mathematician A Sharkovskii, according to which the existence of 3-cycle implies the existence of all other cycles. This theorem makes the study of 3-cycle all the more relevant. The dynamical behavior of the logistic map will be discussed at a introductory level.

Department of Physics, Texas Tech University, Lubbock, Texas

Each of the Group IV elements Si, Ge, and Sn can crystallize in the well-known diamond lattice structure, which is the ground state phase for each. It is less well-known that these elements can also form novel crystalline solids, called clathrates because of structural similarities to clathrate hydrates. These phases were discovered by chemists in the mid-1960’s, but were relatively unknown to materials physicists until the early 1990’s. Group IV clathrates are metastable, expanded volume phases. As in the diamond structure, in the clathrates, the atoms are tetrahedrally coordinated in sp3 covalent bonding configurations with their near-neighbors. However, in contrast to the diamond lattice, the clathrates contain pentagonal rings of atoms and their lattices are open frameworks containing large (20-, 24-, 28-atom) “cages”.  There are two common clathrate varieties: Type I, a simple cubic lattice with 46 atoms per unit cell and Type II, a face centered cubic lattice with 34 atoms per unit cell (136 atoms per cubic cell).
Clathrate cages can contain weakly bound impurities or “guests”, which are usually Group I or Group II atoms. A reason that these materials are interesting is that the choice of guest may be used to tune the material properties. The guests act as electronic donors, but because of their weak bonding, they have only small effects on the host electronic band structures. However, they can vibrate with low frequency vibrational (“rattling”) modes, which can strongly affect the vibrational properties. Some laboratory-synthesized, guest-containing clathrates have been shown to be excellent candidates for thermoelectric applications precisely because the guests only weakly perturb the electronic properties, while strongly affecting the vibrational (heat transport) properties.
            In this talk, the clathrates and their lattice structures will be introduced. The results of calculations of properties of some Si, Ge and Sn-based Type I and Type II clathrates will then be presented. Where data is available, some results will be compared with experiment. The calculations were performed using a density-functional based, planewave, pseudopotential method, and the results include equations of state, structural parameters, electronic band structures, vibrational spectra, isotropic mean-square atomic displacement amplitudes, and thermodynamic properties. Recent results, obtained in collaboration with my former student K. Biswas (now at NREL, Golden, CO) and with my student E. Nenghabi*, will be discussed. Our calculations are motivated by experiments performed by the G. Nolas group at the U. of South Florida. K. Biswas focused on Type II clathrates with filled cages, such as Na16Rb8Si136  and K16Rb8Si136. E. Nenghabi focused on Type I clathrate “alloy” mixtures of Si and Ge, such as Ba8Ga16SixGe30-x, where x is the composition of Si in the “alloy”.

L-3 Communications Infrared Products,
Dallas, Texas

“Thin Film Amorphous Silicon Materials and Devices”

Thin films of silicon and related alloys deposited at low temperatures exhibit a wide range of morphology such as amorphous, nanocrystalline/microcrystalline and polycrysralline, with properties that enable a variety of existing and emerging applications including photovoltaics, thin film transistors, flexible electronics, large area imaging arrays, and MEMS based sensors. A review of deposition, characterization techniques, and properties of amorphous silicon thin films will be given and it will be focused on the correlation of material properties and device performance. In particular, a MEMS application of hydrogenated amorphous silicon (a-Si:H) thin films is in infrared or thermal imaging technology based on micro-bolometer arrays comprised of thin a-Si:H membrane structures suspended over silicon readout integrated circuits. Among the a-Si:H material properties affecting the design and performance of micro-bolometer detectors are electrical conductivity  and its temperature coefficient, thermal and mechanical properties, and interface characteristics. Conductivity near and above room temperature follows an Arrhenius thermally activated dependence,  logs ~ -1/kT, indicative of extended band carrier transport.  At lower temperatures, conductivity is better described by the Mott equation,  logs ~ T-1/4, indicative of a carrier hopping mechanism.  The transport properties are interrelated and optimization is generally applied to the development of microbolometer arrays for thermal imaging applications.

Dr. John V. Foreman,
Research Physicist

“Photoexcited Emission Efficiencies of Zinc Oxide”

Optical properties of the II-VI semiconductor zinc oxide (ZnO) have been studied scientifically for almost 60 years; however, many fundamental questions remain unanswered about ZnO’s two primary emission bands—the exciton-related luminescence in the ultraviolet and the defect-related emission band centered in the green portion of the visible spectrum.  This presentation will provide insight into what factors influence the radiative recombination efficiencies of these emission bands by characterizing their optical properties simultaneously.  Photoluminescence and photoluminescence excitation techniques are used to characterize the thermal quenching behavior of both emission bands in micrometer-scale ZnO powders.  We show that green luminescence excitation and quenching are characterized by localization energies associated with bound excitons,  suggesting that green band photoluminescence intensity directly correlates with the photogenerated exciton population.  The spatial distributions of green-emitting defects and nonradiative traps in these samples are elucidated by an innovative combination of quantum efficiency and time-integrated/resolved photoluminescence measurements.  We take advantage of the drastically different absorption coefficients for one- and two-photon excitations to provide details about the types and concentrations of surface and bulk defects and to demonstrate the non-negligible effects of reabsorption.  A  comparison of results for unannealed and annealed ZnO powders indicates that the annealing process creates a high density of green-emitting defects near the surface of the sample while simultaneously reducing the density of bulk nonradiative traps.  These experimental results are discussed in the context of a simple rate equation model that accounts for the quantum efficiencies of both emission bands.  Finally, we investigate the dependence of luminescence efficiencies on excitation density.  For both femtosecond pulsed and continuous-wave excitations, the green band efficiency is found to decrease with increasing excitation density—from 35% to 5% for pulsed excitation spanning 1-1000 mJ cm-2, and from 60% to 5% for continuous excitation in the range 0.01-10 W cm-2.  On the other hand, near-band-edge emission efficiency increases from 0.4% to 25% for increasing pulsed excitation density and from 0.1% to 0.6% for continuous excitation.  It is shown experimentally that these changes in efficiency correspond to a reduction in exciton formation efficiency.  The differences in luminescence efficiencies for pulsed versus continuous-wave excitation are attributed to changes in the relative rates of exciton luminescence and exciton capture at green defects based on an extended rate equation model that accounts for the excitation density dependence of both luminescence bands.

“Gamma-Ray Bursts: Past, Present and Future”

Gamma-ray bursts (GRBs) are among the most extreme and exciting objects known. I will describe our current state of knowledge on some key aspects of the physics of GRBs, by following the history of the field, in particular the revolution in the 90's and the 2000's made possible by the CGRO, Beppo-SAX and Swift satellites. I will then describe the most recent results obtained by the Fermi satellite. In the second part of the talk, I will focus on some key open questions in the field, which are relevant to many fields in physics and astronomy and are the subject of ongoing extensive studies. Among those are: What is the nature of GRB progenitors? How is energy being extracted? What are the radiative mechanisms, and could GRBs be related to cosmic rays? How can shock waves accelerate particles and produce strong magnetic fields

Department of Physics, University of North Texas, Denton, Texas

“Renewal and Memory: Is there a Contradiction?”

This talk is devoted to the discussion of some recent results, concerning the analysis of brain dynamics, showing the emergence of renewal quakes. Does this conflict with memory? We shall show that it does not and that the crucial events leading the brain dynamics may generate a form of memory much more extended than the ordinary memory generated by extremely slow but stationary fluctuations. We shall show that the abrupt quakes are a natural consequence of criticality, a property shared by decision making models that may have important sociological applications.

Movement and its understanding are vital whether the system under consideration is an electron, a mouse, or a celestial object. Spatial disorder, much more abundant than order, introduces difficulties into the analysis. One method for addressing the difficulties is that of effective medium theory which is fundamental in concept, and converts spatial features into temporal. New developments published recently will be discussed. Random walks and generalized master equations will be mentioned briefly.

Open Public Talk
Tuesday October 13, 2009-7:00pm
ENV 130

Department of Astronomy University of Texas at Austin Austin, Texas

Dr. Shields is a world-known expert who studies giant black holes residing in galactic centers. In his public presentation, he will reveal the mysterious properties of black holes, and describe his exciting new work on black hole interactions. While this subject stands at the forefront of modern astronomy, Dr. Shields has the exceptional ability of bringing this topic down to Earth and appealing to any person regardless of scientific background.

Flyer for Dr. Shields' special colloquium

Tuesday October 13, 2009-3:30pm

Department of Astronomy University of Texas at Austin Austin, Texas

Supermassive black holes with millions to billions of solar masses inhabit the nuclei of most large galaxies. When galaxies merge, the nuclear black holes spiral together. Mergers of spinning black holes can impart a velocity up to 4000 km/s to the product black hole as a result of anisotropic emission of gravitational radiation. Such a "kick" can propel the black hole far from the galactic nucleus or eject it from the galaxy entirely. Observational detection of recoiling black holes would provide verification of the results of numerical relativity and insight into the nature of black hole mergers in galaxies. Possible signatures of recoiling black holes include quasars displaced from the nucleus of their host galaxy, and Doppler shifts of quasar spectral features resulting from the motion of the black hole. I will describe a search for such spectral signatures among thousands of quasars in the Sloan Digital Sky Survey (SDSS). The resulting upper limits for the incidence of high velocity recoils are substantially below theoretical predictions. I will also discuss several proposed examples of quasars with recoiling black holes.

“World-Record White and Monochrome Organic Light-Emitting Diodes Upon Integration of Smart Material Design and Device Physics”

Solid-state lighting based on white organic light-emitting diodes (WOLEDs) promises to provide humongous energy saving due to: (a) low production cost; (b) low power consumption; and (c) realization of 100% internal quantum efficiency (IQE) in multiple device architectures that involve phosphorescent materials. However, challenges remain for stable and highly-efficient blue monochrome OLEDs needed for combination-based WOLEDs, efficiency roll-off due to self-quenching that prevents high performance at practical brightness and current levels, and novel devices capable of emitting white light from a single emitter. The colloquium will show how the approach in the title attained record performance for these challenges in this project supported by a $2.3M contract by the U.S. Department of Energy’s Solid-State Lighting (SSL) program. The latest breakthrough results include 70 lm/W for a blue OLED (vs. 45 lm/W for the previous state-of-the art material), near 100% efficiency for orange OLEDs from neat or highly-doped emissive layers that exhibit “self-sensitization”, and achievement of cool- and warm- WOLEDs with a color rendering index (CRI) up to 82 from a single phosphor, which complies with the Energy Star requirement for white luminaire SSL products.

“Dynamics of the Human Vocal Apparatus and High Precision Spectral Analysis of Human Voice”

The human vocal process consists of soft tissues in the larynx that vibrate due to their interaction with the air stream issuing from the trachea.  The tissues, namely the vocal folds, modulate the air flow and subsequently produce the sound of the human voice.  A significant number of Americans at any given time experience difficulties with their voices in what is generically called “dysphonia.”  This lecture will explore the Myo-elastic Aerodynamic Theory of Phonation, the current theoretical description of how the voice works, and will report on recent findings by Dr. Ling Lu of the Department of Speech and Hearing Sciences and me.  These results hold promise for a non-invasive acoustic screening technique for dysphonia, based on precision spectral analysis of “tokens” of human speech.

Professor and Chair of the Physics Department,
Angelo State University,
San Angelo, Texas

“Physics Teacher Quality”

The Teacher Quality Grant Program of the Texas Higher Education Coordinating Board has a primary goal to improve quality of mathematics and science teaching in Texas.  Execution of these grants and the data they produce are impacted by recent changes in secondary curriculum adopted by the Texas Education Agency in September 2008 and by the College and Career Readiness Standards adopted by the Texas Higher Education Coordinating Board in January 2009.  An overview of this type of grant research will be discussed with the above impacts and changes in science teacher certification in Texas.

“Observing Transiting Exoplanets at the Monroe Robotic Observatory”

Over 300 extrasolar planets (or exoplanets) have been detected in the past two decades, shedding light on planetary formation, the formation of our solar system, and the origins of life in the Universe. However, only about one out of five exoplanets has been observed to transit across the disk of its parent star. Such exoplanet transits enable the most accurate and efficient measurement of the exoplanet mass, which is a key parameter in understanding planetary systems. Clearly, there is a potential for detecting many more of these transiting events thus causing a transformative change in our understanding of planetary science. I will present recent reults of exoplanet transit observations carried out at the UNT Monroe Robotic Observatory, and discuss the prospects of this Observatory in becoming one of the leading centers of exoplanetary observational research.

Professor Chris Littler

“Research in Physics at the University of North Texas”

Dr. Littler will welcome new students to the Physics Department and tell about the department. Then faculty members will introduce themselves and make some brief remarks about  their research interests and group.

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