What happens when two substances are mixed together? Does a chemical reaction occur? If so, which chemical bonds are broken? What new chemical bonds are formed? Can we increase the efficiency of the reaction by changing the conditions under which it occurs? Questions like these lie at the core of chemistry. Addressing them requires understanding, at a fundamental level, how the electrons that bind atoms into molecules rearrange during the course of a chemical reaction and, more subtly, how different molecular environments influence these rearrangements. Therefore, in order to understand the nature of the chemical bond, and to master the chemical reactions by which chemical bonds are fractured and formed, we must uncover the inner lives of electrons.
The physical laws regulating how electrons behave in a molecular environment are encapsulated by the electronic Schrödinger equation. Unfortunately, highly-accurate solutions to the Schrödinger equation are rarely available for molecules containing more than four electrons, while most molecules of interest to chemists contain hundreds, or even thousands, of electrons. This impels the development of approximate models for electronic behavior. Such models are only effective in certain special cases. For example, it is relatively easy to describe cases where the electrons in a molecule move nearly independently, so that the motion of one electron does not affect the other electrons very much. It is also relatively easy to describe cases where the electrons in a molecule are rigidly correlated, so that moving one electron causes the other electrons to move in a nearly deterministic way. The electrons in most chemical substances lie between these two extremes, and developing practical computational methods for these in-between cases is the primary challenge of modern quantum chemistry. In this talk, I will reveal how quantum chemists develop new models for the behavior of electrons in molecules and materials. Some of the new methods are practical even for large molecules containing hundreds of electrons.

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Recent discoveries of rare-earth and alkaline-earth halides with scintillation activators and co-dopants showing excellent properties for spectroscopic gamma radiation detection attract a surge in research activity on their scintillation mechanisms. There is still much to learn about excited states in these materials. Understanding behaviors of the free carries and excitons in the first picoseconds are crucial for determining the speed and nonlinearity of response. Questions remain on whether and when the free electrons are trapped on holes, dopants or defects. The nature of interaction and recombination between the photon-excited species are also important. In this thesis, the crucial early evolution of excited populations is studied with picosecond spectroscopy of optical absorption induced by interband excitation.

We identified the self-trapped exciton (STE) absorption bands in LaBr₃:Ce and CeBr3 samples along with a comparative study on the effects of Ce concentration on the STE absorption decay rate. The dominant scintillation mechanism of both LaBr₃:Ce and CeBr₃ is attributed to dipole-dipole energy transfer from the STE to Ce³⁺ dopant ions on the basis of the transient absorption bands. We identified the charge-transfer excitation of excited Ce^3+* ions for the first time. The population rise time of the Ce^3+* excited states in CeBr₃ (~540 fs) is observed to be faster than in LaBr₃:Ce, and reasons are described. We conclude that our picosecond absorption spectroscopy provides a unique method to assist in the improvement of timing resolution by isolating the rise time of population in the emitting state from the rise time of detected scintillation light, aiming for ultrafast time-of-flight detection.

We also studied the effects of interband excitation on undoped BaBrCl and on BaBrCl doped with Eu and/or Au, as measured by picosecond transient absorption spectroscopy. Aside from the identification of STE absorption bands in BaBrCl samples, we concluded that subsequent dipole-dipole energy transfer from STE to Eu is the dominant energy transfer mechanism. Au co-dopant in BaBrCl:Eu has been found to improve the scintillation light yield, and these transient absorption studies support that the mechanism involves suppression of the concentration of pre-existing halide vacancies.

The Laws of Thermodynamcis, and the properties of Black Holes, are two topics that have long engaged both our imagination and calculational stamina. The formal identification of the area and surface gravity of a black hole horizon as an entropy and a temperature respectively, was made into a physics connection by Hawking’s 1975 calculation that classical black holes radiate quantum mechanical particles. Subsequently the field of black hole thermodynamics has expanded to study black holes in different environments, including black holes with a cosmological constant Λ. Here we will focus on that case of a positive Λ, which plays important roles in cosmology– whether as a GUT (Grand Unified Theory) scale Λ that drives the rapid expansion of the universe during an inflationary epoch, or the milli-eV scale Λ that models the observed dark energy in our universe today. Black holes with Λ > 0 have fascinating properties that are distinct from the asymptotically flat Λ = 0 case, starting with the fact that there are two horizons in the spacetime, one black hole and one cosmological. Hence there are two (generally unequal) temperatures, and two horizon areas that contribute to the total gravitational entropy. Both the mass M and entropy S are bounded between minimum and maximum values. There is a peak in the heat capacity ∂M/∂T as well as in the curve ∂S/∂T, which resemble the Schottky anomaly of a two level system in statistical mechanics. This talk will start with an introduction to black hole thermodynamics and particle production, and then discuss classical and quantum mechanical features of the cosmological black hole system that resemble the physics of a paramagnet.

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Dr. Ledford is this year’s winner of the Distinguished Alumni award. He recently retired as Chief Technology Officer of Emerson Electric Company, one of the world’s leading electronics companies. After graduating from Duke, Dr. Ledford joined Bell Telephone Labs in New Jersey where he worked on microwave communication leading to today’s cell phone communication. Before joining Emerson Electric Dr. Ledford was president and general manager of several divisions of Texas Instruments Inc. including software, digital imaging, enterprise solutions and process automation. Dr. Ledford also generously sponsors scholarships for physics undergraduate majors here at Wake Forest University. He will be speaking about …Emerson Corporation, a global manufacturer of industrial and residential products focusing on the technical and engineering challenges on business in today’s economic climate. Dr. Ledford graduated from WFU with honors in physics (with the assistance of a very young Bill Kerr) and received his Ph.D. in nuclear physics from Duke University.

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Methane gas hydrates buried in the shallow crust of the Earth are often difficult to image with current geophysical techniques.  Understanding their extent in the crustal subsurface is important as they play a role as a future energy source.  In addition, if the clathrates are destabilized from their solid form to a gas, they can enter the atmosphere and affect the climate as methane is a greenhouse gas.  To improve subsurface mapping techniques of gas hydrates, Dr. Bedle and her research group have been approaching the imaging and detection problem by combining rock physics and geophysical seismic techniques.  These methods are additionally enhanced by incorporating new approaches including the use of seismic attributes and machine learning algorithms.  Initial results focused on gas hydrate accumulations in New Zealand will be presented.

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Calculus as taught by mathematicians typically involves a large toolbox of algebraic manipulations. Almost all computations are done using rectangular coordinates and, later, the associated standard basis of unit vectors. Vector calculus as used by physicists, on the other hand, typically involves geometric reasoning, and the frequent use of coordinates and basis vectors adapted to the symmetries that are present. Thermodynamics goes even further, fundamentally altering the notion of “standard coordinates.” These treatments are sufficiently different from each other that they constitute different languages; students are often unable to translate.

Our research group at Oregon State University has been working to bridge this gap between mathematics and physics for more than two decades, primarily by restructuring upper-division physics courses, but also by developing materials for second-year calculus that emphasize geometric reasoning. The Paradigms in Physics project, continuously supported since 1997 by the NSF, has evolved from “merely” designing novel curricula to studying student learning of mathematical concepts such as partial derivatives.

This talk describes several examples of these language differences, the curricular materials we have developed to help students bridge this gap (including an online textbook and a website featuring more than 300 classroom activities), and some of the education research in which our materials are grounded.

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The mechanisms which control the growth of nanocrystals are difficult to investigate because nanocrystals occupy a position awkwardly intermediate between molecules and solids. Two case studies highlight these difficulties and their solution.

(1) Cation exchange is a chemical reaction in which all the cations of a material are replaced by different cations, thus creating a new material. In semiconductor nanocrystals, cation exchange happens extremely fast – many orders of magnitude faster than for macroscopic crystals and far faster than simple size-scaling would suggest. I propose a theoretical mechanism for cation exchange in nanocrystals that reveals a surprising consequence of Coulomb interactions acting at nanometer length scales.

(2) Semiconductor nanostructures take a wide variety of physical forms. One of the most active areas of this research focuses on semiconductor “nanoplatelets,” the name given to nanostructures that are very thin and very wide. An early question asked by researchers was, what causes materials to form these very thin shapes in the first place? The question is even more puzzling when you learn that even materials with an underlying isotropic crystal structure form these extremely asymmetrical shapes. I will propose an explanation of this “kinetic instability” in the growth and show how this theory can be be used by researchers to create new families of nanoplatelet materials.

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In traditional optoelectronics the ability to downscale critical device dimensions has proven extremely successful over the past sixty years in increasing their functionality and performance. These extraordinary developments have been achieved through a virtuous circle of scientific and engineering breakthroughs which have led to the proliferation of information & communication technologies with an extraordinary impact on our daily life and society. However, adopting established manufacturing methods to emerging technologies such as printed optoelectronics, has proven challenging both in terms of technology and economics. This talk will focus on progress being made downscaling emerging forms of large-area optoelectronics through a new fabrication paradigm and their application in a variety of functional devices including, light-emitting nanogap diodes, photo-detectors and rectifying diodes.

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The need for exquisite control of light is ubiquitous in energy-relevant applications, optoelectronics, and information science. In this talk, I will discuss how hybrid materials consisting of distinct sub-lattices and periodically nanostructured materials allow for dramatically enhanced light absorption, emission, and charge carrier generation at various time- and length-scales. I will first focus on hybrid organic-inorganic perovskites. These solution-processed, scalable materials exhibit remarkable optoelectronic properties such as strong light absorption, defect tolerance, and long carrier lifetimes. I will describe how electronic excitations in these materials are coupled to and influenced by the vibrational degrees of freedom of the organic and inorganic sublattices, investigated using an array of optical spectroscopic techniques. The unique soft nature of the lead-halide octahedral framework gives rise to dynamic fluctuations in the electronic bandgap, which distinguishes hybrid perovskites from traditional inorganic semiconductors. Furthermore, strong quantum confinement can be easily imparted to hybrid perovskites with the use of organic spacer-cations, leading to hyperbolic dispersion relation and enhanced light-emitting properties. Beyond solution-processed semiconductors, I will demonstrate how widely-used materials, such as indium-tin-oxide (ITO), can be grown in ordered, nanoscale array form by chemical vapor phase epitaxy to exhibit well-defined localized surface plasmon resonances in the infrared spectral range. The unique band structure and carrier concentration of ITO result in an unusual type of optical nonlinearity that is significantly larger and faster than the noble metal counterparts. I will conclude by discussing how such material platforms open new avenues for infrared molecular sensing, ultrafast optical switching and active photonic devices.

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