|August 26||Hans M. Christen
Oak Ridge National Laboratory
|How Subtle Structural Changes at Epitaxial Interfaces Control Ferroelectricity and Ferromagnetism in Complex Oxides||Hanno Weitering|
|September 2||Labor Day Holiday||No Colloquium||NA|
|September 9||Alex Zunger
University of Colorado
|The Inverse Problem in Nanoscience and in Materials Theory: Find the System that has Desired Target Properties||John Quinn|
|September 16||Ralf Wehlitz
University of Wisconsin
Synchrotron Radiation Center
|Probing Electron Correlations in Aromatic Molecules||Joe Macek|
|September 23||Malcolm Stocks
Oak Ridge National Laboratory
|High Entropy Alloys, a New Class of Structural Materials: What are They, Why do They Form, How do We Predict Them and How Many of Them are Likely to Exist?||Adolfo Eguiluz|
|September 30||John Arrington
Argonne National Laboratory
|Clusters, Correlations, and Quarks: A High-Energy Perspective on Nuclei||Nadia Fomin|
|October 7||John Galambos
Oak Ridge National Laboratory/Spallation Neutron Source
|High Power Hadron Accelerators: Capabilities and Challenges
|October 14||Gregory MacDougall
University of Illinois Department of Physics
|Muons as a Probe of Condensed Matter: a Powerful Complement to Neutron Scattering
|October 21||James B. Hannon
IBM T.J. Watson Research Center
|Carbon Nanotubes from Lab to Fab||Hanno Weitering|
|October 28||Ian Fisher
Geballe Laboratory for Advanced Materials and Department of Applied Physics
|Electronic Nematic Phases in High Temperature Superconductors||Jan Musfeldt|
|November 4||Geoff Greene
|The Beta Decay of the Neutron||NA|
|November 11||Susan Gardner
University of Kentucky Department of Physics
|Prospects for Resolving the Puzzle of the Missing Antimatter
|November 18||Andrew McWilliam
The Carnegie Institution for Science
|Nucleosynthesis and Chemical Evolution from Las Campanas Observatory||Kate Jones|
|November 25||Peter Mohr
National Institute of Standards and Technology
|The Proton Radius Puzzle: An Unsolved Mystery
|December 2||John Katsaras
Oak Ridge National Laboratory
|The Neutron- and Computation-Based Biomembranes Program at ORNL||Joe Macek|
How Subtle Structural Changes at Epitaxial Interfaces Control Ferroelectricity and Ferromagnetism in Complex Oxides
Perovskite complex oxides (i.e., ABO3 compounds) exhibit a strikingly broad range of physical behaviors – in fact, just about any technologically relevant functional property can be found within this family of materials, ranging from superconductivity to ionic conductivity, from ferroelectricity to magnetism, from piezoelectric to electro-optic coupling, etc. Many of these properties are profoundly modified when the material is placed in direct contact with a different perovskite, making it possible to conceive new types of devices to utilize these functionalities. Synthesis techniques including molecular beam epitaxy and pulsed-laser deposition (PLD) have been developed that make it possible to stack perovskite blocks almost as easily as Legos. The resulting well-controlled epitaxial interfaces not only allow us to develop new devices, but also to study the fundamental mechanisms that are at the origin of the complex behaviors of the constituents. While this is perhaps best illustrated by much-discussed example of the electronically conducting interface that forms between two insulators, LaAlO3 and SrTiO3, numerous other examples have been explored. In this presentation, I will use PLD-grown structures to illustrate the importance of structural effects, and show that they go far beyond elastic deformations due to epitaxial strain. For the case of multiferroic BiFeO3, I will show how strain induces polymorphic phase transitions and how the interplay between the resulting phases leads to fascinating properties. For the case of superlattices of SrTiO3 and antiferromagnetic LaMnO3 I will show that small distortions in the tilt pattern of the relatively rigid network of oxygen octahedra within the perovskite unit cell results in the formation of a ferromagnetic interface. These examples illustrate that structural effects can play a dominant factor, perhaps even in systems for which electronic reconstruction has previously been thought to be the key mechanism.
The Inverse Problem in Nanoscience and in Materials Theory: Find the System that has Desired Target Properties
Condensed matter physics and material research has historically often proceeded via trial-and-error or even accidental discoveries of materials with interesting physical properties, including new ferromagnets, superconductors, magneto–resistors, transparent-conductors, carbon nanotubes, etc. The question posed in this talk is: does it make sense instead to first declare the physical property you really want, then find which structure/material has this property? I will describe recent advances in the way quantum-mechanical electronic structure calculations have been combined with biologically-inspired (“genetic”) evolutionary approaches to scan a truly astronomic number of atomic configurations in search of the one that have desired, target electronic properties (“Material Genome Initiative”). Recent examples of such “Inverse Design” in the areas of nanostructures, magnetism, semiconductors and spectroscopy will be mentioned. This work was also borne out of the recognition that many materials that can be expected to exist, are in fact missing from the compilations of all materials previously made. Are they missing for a good reason (i. e, they are intrinsically unstable), or did people did not get around to making them yet, but they could have interesting properties? I will describe the way modern “first principles thermodynamics” can address this question, and in the process discover quite a few inorganic structures and materials that should exist, but are yet undiscovered. Experimental efforts to make such materials are underway in the newly formed “Energy Frontier Research Center on Inverse Design.”
Probing Electron Correlations in Aromatic Molecules
Electron correlations can be found in various fields of physics whenever we have to go beyond the independent particle model. A convenient method to study electron correlations in gas-phase atoms and molecules is to measure the probability to remove two electrons simultaneously with a single photon (called double photoionization) from the sample. Because a single photon can interact with only one electron, the removal of two electrons is due to electron correlations. I will present our recent results on double photoionization of aromatic hydrocarbons over a broad range of photon energies. Our goal is to find systematic trends as the structure of our different sample molecules changes. Questions that will be addressed in the talk are: How differ molecules from atoms regarding double photoionization? How does the structure of a molecule affect the double-photoionization process? Which mechanisms contribute to double photoionization?
High Entropy Alloys, a New Class of Structural Materials: What are They, Why do They Form, How do we Predict Them and How Many of Them are Likely to Exist?
The development of metallic alloys is arguably one of the oldest of sciences, dating back at least 3,000 years. It is therefore very surprising when a new class of metallic alloys is discovered. High Entropy Alloys (HEA) represent such a class, a class that is receiving a great deal of attention due to their unusual combinations of strength, ductility, thermal stability, corrosion and wear resistance that make them candidates for technological applications. The term “high entropy alloys” typically refers to alloys that are comprised of 5, 6, 7 or even more elements, at or near equi-atomic composition, that form simple solid solution alloys on simple underlying lattices such as FCC and BCC. The appellation “High Entropy Alloys” refers to an early conjecture that these unusual systems were stabilized as solid solutions by the high entropy of mixing associated with the large number of components. In this presentation, I will present a simple model that provides answers to a number of fundamental questions posed by the very existence of these alloys, not the least of which are: what are the driving mechanisms for their unexpected stability, which combinations of elements can give rise to HEAs, and how does the number of possible alloys depend on the number of constituents species? The approach is based on ab initio “high through-put” calculations of the enthalpy of formation of simple binary phases. In particular we have developed a 31-element “enthalpy matrix” that allows one to discriminate between combinations of elements that form single-phase solid solution HEAs and similar combinations that do not. Despite the increasing entropy, our model predicts that the number of potential single-phase multicomponent alloys that can form decreases with an increasing number of components. Interestingly, out of more than two million possible 7-component alloys considered, fewer than twenty single-phase alloys are likely. If time permits, I will also present results of calculations that indicate HEA comprising combinations of the 3d-transition metals Cr, Mn, Fe, Co and Ni which may also possess unusual magnetic properties and provide predictions of nature of the magnetic properties in the hope of stimulating experimental investigations of them.
This work was performed in collaboration with Claudia Troparevsky, James Morris, Paul Kent, Andrew Lupini, Markus Daene, Junqi Yin, Markus Eisenbach, Aurelian Rusanu, and Khorgolkhuu Odbadrakh. This work was supported by the Materials Sciences and Engineering Division of DOE-BES and the “Center for Defect Physics of Structural Materials,” a Department of Energy, “Energy Frontier Research Center.”
Clusters, Correlations, and Quarks: A High-Energy Perspective on Nuclei
The dramatic separation of energy scales between the interactions that bind atoms, nuclei and quarks yields remarkable simplifications in the study of the nature of matter. Quark structure is essentially irrelevant when dealing with protons and neutrons in nuclei, while the rich details of nuclear structure all but disappear when studying atomic interactions.
Exceptions to this factorized picture of matter allow for interesting measurements of nuclear structure using either very low or very high energy probes, and may also lead to cases where clustering phenomena in nuclei can impact the quark distributions probed at high energies. I will begin by giving some examples of phenomena which overlap these disparate energy scales, and then focus on the case of extremely high-density clusters in nuclei.
High Power Hadron Accelerators: Capabilities and Challenges [Presentation Slides]
Throughout much of the past century, particle accelerators have been a critical tool for enabling high energy and nuclear physics studies. As more rare processes are pursued, higher power accelerators are needed to explore these events. High power accelerators are also used as drivers for producing spallation neutrons used in neutron scattering, and future applications include drivers of sub-critical fission systems for energy production and transmutation of fission product wastes. The Spallation Neutron Source at Oak Ridge National Laboratory is one of the world’s highest power proton accelerators, capable of over 1 MW beam power. The high power operation experience will be discussed, along with plans for future high power (multi-MW) accelerators under consideration around the world. High power operational challenges, such as managing beam loss and target survivability will also be discussed.
Muons as a Probe of Condensed Matter: a Powerful Complement to Neutron Scattering [Presentation Slides]
Muon spin rotation (μSR) is a technique which uses the parity violating decay of the muon to measure magnetic fields and fluctuations inside condensed matter systems. Over the past forty years, it has made meaningful contributions to the fields of magnetism, superconductivity, quantum diffusion, semiconductor physics, and chemistry. The recent development of low-energy muon beams, make μSR one of the only depth-resolved probes of magnetism at surfaces or in thin films. Partially as a result of this new development, the construction of new μSR facilities is being discussed in China, Korea and the United States, while capacity is being expanded in existing sources. I will give an introduction to μSR, and discuss the scientific questions that can be address with the technique. Example data will be given, with particular attention paid to questions of interest to the neutron scattering community and the complementary information obtained by muons and neutrons.
Carbon Nanotubes from Lab to Fab
It is now widely appreciated that electronic devices based on carbon nanotubes (CNTs) have the potential to outperform conventional silicon devices. As the challenges associated with scaling silicon technology mount, CNTs are being seriously considered as an alternative, or successor, to silicon in high-performance logic applications. However, in order to create a viable high-performance CNT technology, daunting and unique integration challenges must be overcome. These include isolation of semiconducting CNTs, selective placement of CNTs from solution, and passivation. In this talk I will describe recent progress, made at IBM and elsewhere, on solving these challenges. I will highlight the device performance requirements for a competitive CNT technology, which guides our integration strategy. Finally, I will discuss the areas where fundamental research is needed in order to improve device performance.
Electronic Nematic Phases in High Temperature Superconductors
Borrowing language from the field of soft condensed matter, electronic nematic phases in crystalline systems break a discrete rotational symmetry of the crystal lattice without further breaking translational symmetry. In this talk I'll outline methods to measure an associated quantity, the nematic susceptibility, which diverges towards a nematic phase transition. These measurements directly reveal the presence of an electronic nematic phase transition in a family of high temperature superconductors, and an associated quantum phase transition near optimal doping.
The Beta Decay of the Neutron
While neutrons within nuclei may be stable, the free neutron is unstable against beta decay and has a mean lifetime of ~15min. Free neutron beta decay is a rather simple nuclear process that is uncomplicated by the many the many body effects that are present in the decay of heavy nuclei. As a result, it can be understood in terms of rather simple fundamental weak interaction theory. As the "prototype" for all nuclear beta decays, the free neutron lifetime is a fundamental parameter whose value is important not only in nuclear physics, but also in astrophysics, cosmology, and particle physics. I will give an introduction to the theory of weak nuclear decay and discuss the importance of the neutron lifetime as a parameter in the Big Bang. A discussion of the experimental strategies for the measurement of the neutron lifetime will be given and I will present a new result recently obtained by the University of Tennessee Group at NIST Cold Neutron Research Facility in Gaithersburg Md.
Prospects for Resolving the Puzzle of the Missing Antimatter [presentation slides]
A cosmic excess of baryons (over antibaryons) is well-established, but the mechanism by which it is generated is not. It has long been thought that an explanation for this missing antimatter can be found within particle physics, though the Higgs discovery establishes once and for all that physics beyond the Standard Model is necessary to explain it. I will review the broad scope of the discussed resolutions and show how various, low-energy, precision measurements which search for the violation of discrete symmetries, most notably matter-antimatter symmetry CP and baryon number B, can help resolve Nature's favored pathway.
Nucleosynthesis and Chemical Evolution from Las Campanas Observatory
Las Campanas observatory houses two of the largest optical telescopes in the world. Coupled with an efficient high-resolution spectrograph, these telescopes permit detailed chemical abundance measurements of individual stars in the nearest members of the Local Group of galaxies.
I discuss a few ways in which the chemical composition of the envelopes of old stars provides constraints on the astrophysical sites of nucleogenesis, stellar structure and evolution, and the evolution of galaxies.
The Proton Radius Puzzle: An Unsolved Mystery [presentation slides]
In July 2010, a paper in Nature reported a measurement made in Switzerland of the Lamb shift in muonic hydrogen, a negative muon bound to a proton, that may be considered a measurement of the proton radius. This value for the radius differs by seven standard deviations from the 2010 CODATA recommended value. The disagreement is a puzzle, and could conceivably turn out to be a fundamental shortcoming of quantum electrodynamics (QED). This would also be bad news for the fundamental constants, because many of the values rely on the assumption that the theory is correct.
The Neutron- and Computation-Based Biomembranes Program at ORNL
Understanding biological membranes — their structure, dynamic behavior and function — stands as a grand scientific challenge. Membranes also happen to be intrinsically disordered assemblies, making them unsuitable for study by classical structural methods, such as x-ray crystallography. As a result, multiple experimental approaches must be applied to address the challenges they present. Recognizing the potential impact of biomembranes science, ORNL has made strategic investments toward the creation of a world-leading program in neutron- and computation-based biomembranes research. This program is focused on model systems that can be interrogated through neutron scattering, as well as other analytical methods, and are in most cases amenable to molecular simulations. Ultimately, the program aims to understand, predict and control the emergent behaviors in functional membrane systems. Today’s seminar will present recent examples, which will illustrate the science presently undertaken by the Biomembranes Program.
Knoxville, Tennessee 37996 | 865-974-1000
The flagship campus of the University of Tennessee System