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Spring 2005 Physics Colloquium ScheduleUnless otherwise noted, the physics colloquia are held in Room 307 of the Science and Engineering Research Facility. Refreshments are served at 3:00 p.m. with the talk following at 3:30. Abstracts are included below the schedule. The ORNL Physics Division Seminar Schedule might also be of interest. Professor Witek Nazarewicz is chair of the colloquium program. He may be contacted via e-mail at: witek@utk.edu. AbstractsJanuary 24: A New Spin on SemiconductorsHanno Weitering (UT Physics/ORNL) Semiconductor spin-electronics or “spintronics” is a burgeoning field in condensed matter research. It has presented the research community with numerous scientific challenges, ranging from questions of how to understand and control spin-dependent electrical transport through interfaces to the more basic materials issues, specifically those that relate growth and processing with magnetism and electrical transport. The promise is a whole new generation of spin-based electronic devices with enhanced functionality, higher speeds, and reduced power consumption.Dilute magnetic semiconductors (DMS) are particularly interesting spintronic materials. They can easily be integrated into semiconductor hetero-structures while “spin injection” from a DMS source electrode may be very efficient because of the natural impedance match to the semiconductor channel. We explored the possibility of making germanium ferromagnetic by doping Ge with manganese, both randomly and in so-called "digital" arrangements, using molecular beam epitaxy. Ferromagnetism in randomly-doped Ge is dictated by impurity band conduction and the percolation of “bound magnetic polarons”. This not only distinguishes Mn-doped Ge from the more familiar Group III-V DMS (making it the first example of a “strongly coupled” DMS) but also indicates interesting analogies with other doped magnetic materials, including the colossal magneto-resistance materials. Finally, I will show how controlled placement of the magnetic dopants can lead to remarkable enhancement of the ferromagnetic ordering temperature. The established compatibility of germanium and silicon may one-day allow integration of magnetism and silicon-based electronics. January 31: Detecting Rare Isotopes: Pushing the limits of Accelerator Mass SpectrometryAlfredo Galindo-Uribarri (ORNL) Accelerator Mass Spectrometry (AMS) is an analytical technique to detect rare isotopes at unprecedented level of sensitivity. AMS evolved from detection methods and techniques developed in nuclear physics and was introduced first in 1977 to measure the radioactive isotope of carbon, 14C, by counting individual atoms in milligram samples to establish when an organism died. Since then, many other rare or long-lived isotopes have been utilized and AMS now has applications in almost every environmental field. I will review modern developments in this field, giving examples of applications that range from neutrino detectors to Homeland Security. I also will describe future opportunities for AMS available at the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory. Recently, with the aid of the 25-MV tandem accelerator (the highest voltage accelerator in the world) we have pushed the AMS technique to its limits in measuring the lowest isotopic ratio of 36Cl/Cl ever. Surprisingly, we have found variation in the content of this isotope (fallout product) on various seawater samples taken from around the world. Finally, I will discuss how two fields of research with apparently little in common — Radioactive Ion Beam Physics and AMS— complement each other in techniques to isolate the isobars of interest. February 7: The Neutrino Revolution in Cosmology and Stellar CollapseGeorge Fuller (University of California, San Diego) In just the last few years many properties of the ghost-like neutrinos have been revealed by experiment and observation. Several other key properties remain elusive. The stakes are high, however, because although neutrinos individually have exceedingly weak coupling with matter, they more than make up for this in the early universe and in the collapsing cores of supernovae by shear numbers. In fact neutrinos dominate all of the important regimes of these environments and play a pivotal role in the creation of the elements. Here we will confront the cabal of gravitation, entropy, and neutrinos and discuss its implication for cosmology and our models for supernovae.February 14: The Mystery of MatterStefan Spanier (UT Physics) The answer to the fundamental question: "Why is our Universemade up of matter and not antimatter?" might be found in the laws that govern elementary particles. The BaBar experiment at the electron-positron collider of the Stanford Linear Accelerator Center (SLAC) measures asymmetries in the decays of particles and anti-particles, an important ingredient to explain the anti-matter disappearance in the evolution of the Universe. Last year BaBar observed one of the largest asymmetries ever in particle decays. Most of the measured differences are in very good agreement with our present understanding, the Standard Model of Particle Physics. Unfortunately, this also makes them too small to play a significant role in the matter-antimatter asymmetry we observe today. But some of the reactions reveal disagreement with predictions possibly hinting to New, not yet explored Physics Laws. February 21: Laboratory CosmologyDaniel Wolf Savin (Columbia Astrophysics Laboratory) Atomic and molecular physics during the early Universe has become a topic of major significance over the last decade. This is due to the recent revolution in our understanding of the cosmological evolution of the Universe. Recent observational, theoretical, and computational advances in cosmology have shifted our understanding of the early Universe from qualitative to quantitative. Concurrent with this shift has been a deeper realization of the cosmological importance of atomic and molecular processes. For example, studies of primordial galaxy and first star formation require accurate models of the hydrogen chemistry during this epoch. As another example, interpreting observations of heavy elements in the high-redshift intergalactic medium (IGM) can be used to constrain the chemical evolution of the early Universe and the formation of the stars which produced these elements. But to do this requires a reliable understanding of the underlying atomic and molecular processes which determine the ionization balance in the IGM. Surprisingly, an accurate understanding is often lacking of the atomic and molecular physics needed to advance these and other cosmological studies. Many of the cosmologically important atomic and molecular processes occur in energy regimes which can be theoretical, computationally, and experimentally challenging or even inaccessible. Here I will report on a series of recent laboratory measurements, theoretical calculations, and modeling studies which we have carried out in order to improve our knowledge of atomic and molecular physics under cosmological conditions and to thereby increase our understanding of the early Universe.February 28: From Einstein to String TheoryGeorge Siopsis (UT Physics) It has been 100 years since Einstein's "annus mirabilis" which started a string of discoveries that changed our way of thinking about fundamental concepts such as space and time, mass and energy, gravity, and helped establish the quantum nature of our Universe. Einstein's work left important questions unanswered, such as how to reconcile gravity and quantum mechanics and unify all forces of Nature. Today, we have a theory which provides answers to these questions: String Theory. This talk will review the successes of String Theory, how it compliments and modifies Einstein's ideas, and give a flavor of today's on-going research. It is amazing that a century after Einstein's first publications, "cutting edge" is still defined by the problems he could not solve.March 7: The Sokal Affair and the History of ScienceDenise Phillips (UT Department of History) In 1996, the physicist Alan Sokal sparked a vigorous debate after he revealed that he had managed to get his parody of contemporary cultural studies (an article entitled "Transgressing the Boundaries: Toward a Transformative Hermeneutics of Quantum Gravity") published in the scholarly journal Social Text. Sokal hoped that his parody would help to stem what he saw as a dangerous trend towards irresponsible, antiscientific relativism within the humanities. The talk will give an overview of the "Sokal Affair" (as it has come to be called), and then will discuss Sokal’s criticisms in light of recent research in the history of science.March 14: A Microscopic View of Piezoelectric OxidesDavid Singh (UT/ORNL Condensed Matter Sciences Division) This talk presents a case study on the application of computer simulations to find new materials for given applications, here transducers. Piezoelectric oxides, particularly certain Pb containing perovskites, are widely used for electro-mechanical transducers, that is, devices that convert sound into electrical energy and vice versa. Applications range from medical ultrasounds to SONAR. In this talk I will overview the physics of these materials and their piezoelectric properties. As will be seen their performance depends on a remarkable balance between different lattice instabilities of the perovskite structure. These in turn depend on the chemistry of Pb, which poses a challenge, because environmental regulations mandate the removal of Pb from electronics. We have been using density functional calculations to understand the details of the lattice instabilities in microscopic terms. This has yielded partial understanding of how to control them, which is summarized in four design rules.March 28: Designed Emergent Functionality from ComplexityMichael L. Simpson (UT Department of Materials Science and Engineering/ORNL Molecular-Scale Engineering and Nanoscale Technologies Research Group) Research at the interfaces between life and physical sciences is connected by several common themes that include complexity; robust function in an environment of large inherent stochastic fluctuations in small systems; and the controlled synthesis and directed assembly of nanoscale materials, devices and systems. In the context used here, the term complexity describes systems where the interconnections between the fundamental components play a greater role in process end points than the specific function of these components. This talk focuses on the control of a complex process for the controlled synthesis and directed assembly of nanoscale structures and systems. Specifically, the synthesis of carbon nanofibers (CNFs) with a particular emphasis on the use of CNFs to make molecular-scale interconnections to and mimic the functionality of cellular systems is described. The focus of this part of the talk is on the roles of a number of process interactions (e.g. catalyst-substrate; catalyst-nanofiber; growth conditions-nanofiber-catalyst) that can be manipulated to control CNF properties. A second focus will be the role of stochastic fluctuations, which become more pronounced in small systems. As a model system where robust functionality is maintained in a very noisy environment, the inherent noise of genetic systems is considered both analytically and experimentally.April 4: The Next Step in Fusion Energy: ITER and the Challenge of Predicting it's BehaviorDon Batchelor (ORNL Fusion Energy Division) The next major step being planned for the development of fusion energy is construction of ITER, a tokamak device in the $5B class, capable of producing several hundred megawatts of fusion power. The plasmas in such devices are extremely far from thermal equilibrium and support a vast number of physical processes that must be controlled and coordinated to successfully achieve the conditions required for fusion. Simulation is a key element in the research program needed to understand experimental results from devices and compare these results to theory, to plan and design experiments on the devices, and to invent and evaluate new, higher performing confinement concepts. There are a number of fundamental computational challenges in such simulation: extreme range of time scales - wall equilibration time/electron cyclotron time O(1015), extreme range of space scales - machine radius/electron gyroradius O(104), extreme plasma anisotropy - mean free path in magnetic field parallel/perpendicular O(1010), strong non-linear coupling, sensitivity to geometric details, and high dimensionality. The talk will outline the dominant scientific issues for successful creation of burning plasma, will describe the various approaches to fusion plasma simulation and progress toward bringing together the various models so as to treat the plasma more self-consistently. April 11: Atomic Chains: From Low-Dimensional Electrons to the Limits of Data StorageFranz Himpsel (University of Wisconsin) One-dimensional physics is particularly elegant because of its mathematical transparency. However, it is not easy to realize a one-dimensional system experimentally. Recently, it has become possible to produce arrays of gold chains at silicon surfaces by self-assembly and to control their dimensionality and band filling. Angle-resolved photoemission reveals exotic band structures and Fermi surfaces, including a fractional electron count of 8/3 that is explained by a low-dimensional version of the doping in HiTc superconductors. These structures can be used as atomically-precise tracks for a memory where a bit is stored by the presence or absence of a single silicon atom. This toy memory is used to test the fundamental limits of data storage and to see how well storage in silicon compares to storage in DNA. April 18: High Tc Superconductivity and Your Cell PhoneDouglas Scalapino (University of California, Santa Barbara) Now, almost two decades after Bednorz's and Muller's discovery of the high Tc cuprates, these unique materials are finding applications. Here I will discuss the physics, material science and engineering that have lead to the development of thin film cuprate superconducting filters for their application to cellular communication.
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