Fall 2008 Physics Colloquium Schedule
Unless 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. The ORNL Physics Division Seminar Schedule might also be of interest. Professor Stefan Spanier is chair of the colloquium program. He may be contacted via e-mail at: spanier@utk.edu.
Physics Colloquium Webcasts
| Date | Speaker and Title |
|---|---|
| August 25 | Dirk Morr, University of Illinois at Chicago "Collective modes in unconventional superconductors" |
| September 1 | Labor Day Holiday |
| September 8 | Phil Andrews, NICS: Oak Ridge National Laboratory "NSF Supercomputing: from the dark ages to Kraken, and what it can do for you!" |
| September 15 | Paul Snijders, Oak Ridge National Laboratory "One-dimensional atom wires: structural fluctuations and electronic instabilities studied with scanning tunneling microscopy" |
| September 17 | Special JIAM Colloquium: Pengcheng Dai, University of Tennessee |
| September 22 | Lance Cooper, University of Illinois Urbana-Champaign "Exploring emergent phenomena and "highly tunable" behavior in strongly correlated materials" |
| September 29 | Dam Thanh Son, University of Washington "Viscosity, black holes, and relativistic heavy ion collisions" |
| October 6 | Jolie Cizewski, Rutgers University "Applying Nuclear Physics to Address Challenges in National Security" |
| October 13 | Thomas Papenbrock, University of Tennessee "Frontiers of the nuclear quantum many-body problem" |
| October 20 | Mark Kasevich, Stanford University "Atomic interferometry for detection of gravity waves" [.pdf file of .ppt presentation] |
| October 27 | Leopoldo Garcia-Colin, Universidad Autonoma Metropolitana-Iztapalapa in Mexico "Transport processes in dilute plasmas" |
| November 3 | Witek Nazarewicz, UT/ORNL "Science of rare isotopes: connecting nuclei with the universe " |
| November 10 | Ivan Bozovic, Brookhaven National Laboratory "Insights in high-TC superconductivity from atomic-layer engineering" |
| November 17 | Warren Grice, Oak Ridge National Laboratory "The entanglement your mother never warned you about" |
| November 24 | Denis Dujmic, Massachusetts Institute of Technology "Toward a dark matter telescope" |
Dirk Morr, "Collective modes in unconventional superconductors"
Identifying the collective mode responsible for the emergence of high-temperature superconductivity holds the key to understanding the unconventional properties of the cuprate superconductors. In this talk, I focus on the most likely candidate for the pairing mode, a collective spin mode. In the superconducting state, this mode gives rise to one of the most puzzling phenomena in the high-temperature superconductors, the resonance peak observed by inelastic neutron scattering experiments. I will argue that this peak arises from a feedback effect of superconductivity on the magnetic excitation spectrum - a feedback effect that reflects the dx2-y2-wave symmetry of the superconducting gap as well as the topology of the Fermi surface. I will present a number of theoretical studies which show that in the optimally and overdoped cuprates, the experimentally measured momentum and frequency dependence of the resonance peak is consistent with a spin exciton nature of the resonance, i.e., a particle-hole bound state inside the spin-gap. Moreover, I will discuss a new formalism for computing the spin susceptibility, and in particular the resonance peak, directly from experimental single particle Green’s functions derived from angle resolved photoemission spectroscopy. Finally, I will demonstrate that this mode leads to characteristic signatures in the local density of states around impurities that distinguish it from non-magnetic modes.
Phil Andrews, "NSF Supercomputing: from the dark ages to Kraken, and what it can do for you!"
Phil Andrews has been involved in supercomputing from the late '70s to the present, when he is now project director for the latest NSF supercomputer to enter production: the University of Tennessee's Kraken system, scheduled to become the world's fastest academic supercomputer next year. He will mention some of his research, beginning at the Princeton Plasma Physics Laboratory, moving through the creation of Mosaic, the first widely used graphics browser, and into medical supercomputing. But the talk will mainly detail the progress, paradigm shifts, successes and other outcomes, from thirty years of National Science Foundation supercomputing, leading up to the current TeraGrid environment with enormous capability for researchers.
Paul Snijders, "One-dimensional atom wires: structural fluctuations and electronic instabilities studied with Scanning Tunneling Microscopy"
The physics of one-dimensional systems is particularly elegant because of its mathematical transparency. From the experimental side, however, it remains difficult to find systems that are sufficiently decoupled from higher dimensions to really form one-dimensional systems. One way to fabricate and study one-dimensional systems is to utilize self-organization of metal adsorbates at vicinal, or stepped, silicon surfaces. Depositing metal atoms on vicinal surfaces tends to result in self-organized arrays of atom wires - chains of only one atom diameter. We have studied the two major aspects affecting the properties of these atom wire arrays. First, the low dimensionality of the system results in large thermodynamic fluctuations. These fluctuations limit the degree of order that can be attained in self-organized atom wire arrays. We have experimentally (Scanning Tunneling Microscopy and Low Energy Electron Diffraction) studied the ordering of missing atoms (vacancies) in atom wire arrays. We also developed a novel theoretical approach to model these results. The second aspect we studied is the tendency to electronic stabilities of metallic atom wires. Their one-dimensional Fermi surface often drives the formation of a symmetry breaking Charge Density Wave (CDW) that induces a gap in the Density of States, destroying the metallicity of the wires at low temperature. Using Scanning Tunneling Microscopy and Spectroscopy, we discovered a complex sequence of different competing CDWs within a single atom wire. These observations could be explained using the bandstructure of the high symmetry phase, and an interband doping mechanism attributed to defects present in the wires.
Lance Cooper, "Exploring emergent phenomena and "highly tunable" behavior in strongly correlated materials"
Abstract: Strong coupling between the charge, spin, and lattice degrees of freedom in transition metal oxides and other materials spawns numerous interesting properties, including diverse phase behavior as functions of temperature, magnetic field, and pressure, and exotic phenomena, such as colossal magnetoresistance, charge/orbital ordering (COO), exotic superconductivity, and quantum (T=0) phase transitions. In this talk, I'll try to illustrate the remarkably diverse phenomena and exotic phase behavior exhibited by these complex materials, as seen using magnetic-field and pressure-tuned light scattering to explore the evolution of various quantum phases through low temperature phase transitions. Among the phenomena and materials I'll survey: (i) the pressure- and doping-induced collapse of the charge density wave state in TiSe2, which results in various unconventional emergent phases; (ii) field- and temperature dependent melting of charge- and orbital-order in the manganese oxides, where we observe behavior analogous to field-induced crystallization of an electronic solid; (iii) field-induced structural and conducting phase changes in Ca3Ru2O7, in which strong spin-orbit coupling causes the tunability of the material to be dependent upon both field strength AND direction; and (iv) field-induced structural phase changes near the incommensurate-commensurate phase transition of multiferroic TbMnO3, where a magnetic field induces a rearrangement of the ferroelectric polarization direction. All of these examples illustrate how both "highly functional" behavior and novel emergent phenomena can result from the close competition among several interactions in correlated materials.
Dam Thanh Son, "Viscosity, black holes, and relativistic heavy ion collisions "
Viscosity is a very old concept which was introduced to physics by Navier in the 19th century. However, in strongly coupled systems viscosity is extremely difficult to compute ab initio. In this talk I will describe some recent surprising developments in string theory which allow one to easily compute the viscosity in a class of strongly interacting relativistic quantum field theories. I will describe efforts to measure the viscosity and other physical properties of the quark gluon plasma created at the Relativistic Heavy Ion Collider.
Jolie Cizewski, "Applying nuclear physics to address challenges in national security"
Should the unthinkable occur and a nuclear device is exploded by a terrorist group or rogue nation, nuclear forensics can be applied to help identify the source of the material. In nuclear forensics, the recovered material is compared with information in databases or expectations from models of nuclear devices. To provide the base line data to help identify the source and composition of the nuclear material, there is a need to understand better the nuclear reaction processes that could occur. A particular challenge is understanding nuclear reactions on radioactive isotopes, for example those produced in the fission of uranium or plutonium. Recently, we have begun a program to measure nuclear reactions on fragments following uranium fission using accelerated radioactive beams of the short-lived isotopes. The present talk will describe this new research program and present the first results.
Thomas Papenbrock, "Frontiers of the nuclear quantum many-body problem"
Nuclear structure theory has made significant advances over the past decade. The inter-nucleon interaction can now systematically be derived and improved within effective field theory, and coupled- cluster calculations have been pushing the frontier of ab-initio calculation from light to medium-mass nuclei. There is a worldwide effort to improve microscopic mass models based on nuclear energy- density functional theory, and I will present new ideas and recent results obtained in this field. Finally, I will also touch upon interdisciplinary aspects of nuclear theory such as Fermi gases with large scattering length and chaos in atomic nuclei.
Mark Kasevich, "Atom interferometry"
Atom de Broglie wave interferometry has emerged as a tool capable of addressing a diverse set of questions in gravitational physics, and as an enabling technology for advanced sensors in geodesy and navigation. This talk will review basic principles, then discuss recent applications and future directions. Scientific applications to be discussed include measurement of G (Newton's constant), and tests of the Equivalence Principle and post-Newtonian gravity. Technological applications include development of precision gryoscopes and gravity gradiometers for advanced inertial navigation systems and gravity anomally surveys. The talk will conclude with speculative remarks looking to the future: Can atom interference methods be used to detect gravity waves? Can non-classical (entangled/squeezed state) atom sources lead to meaningful sensor performance improvements?
Leopoldo Garcia-Colin, "Transport processes in dilute plasmas"
The purpose of this talk is to show how the full kinetic theory of a dilute inert plasma can be derived from Boltzmann´s equation. The results may be regarded as providing the microscopic basis of the Linear Classical Irreversible Thermodynamics of these systems in the presence of magnetic fields. The calculations lead to explicit analytical expressions for all transport coefficients. Some applications to astrophysical problems will be mentioned if time allows.
Witek Nazarewicz, "Science of rare isotopes: connecting nuclei with the universe"
Understanding nuclei is a quantum many-body problem of incredible richness and diversity and studies of nuclei address some of the great challenges that are common throughout modern science. Nuclear structure research strives to build a unified and comprehensive microscopic framework in which bulk nuclear properties, nuclear excitations, and nuclear reactions can all be described. A new and exciting focus in this endeavor lies in the description of exotic and short lived nuclei. The extreme proton-to-neutron asymmetry of these nuclei isolates and amplifies important features of nuclear many-body open quantum systems.
The fields of nuclear physics and astrophysics provide the link between our understanding of the fundamental constituents of nature and explaining the matter of which we and stars are made. Studies of rare isotopes elucidate fundamental questions in this area.
In this talk, experimental and theoretical advances in rare isotope research will be reviewed in the context of the main scientific questions. Particular attention will bo given to the worldwide radioactive beams initiatives and to the progress in theoretical studies of nuclei due to the advent of terascale computing platforms.
References:
Rare-Isotope Science Assessment Committee Report, The National Academies Press
http://books.nap.edu/openbook.php?isbn=0309104084
"Computing Atomic Nuclei", SciDAC Review, Winter 2007
http://www.scidacreview.org/0704/pdf/unedf.pdf
Ivan Bozovic, "Insights in high-TC superconductivity from atomic-layer engineering"
Using molecular beam epitaxy, we synthesize atomically smooth HTS thin films, multilayers and superlattices.1 Such heterostructures enable novel ex-periments that probe the basic physics of HTS. For example, we have estab-lished that HTS and anti-ferromagnetic phases separate on Ångstrom scale, while the pseudo-gap state apparently mixes with HTS over an anomalously large length scale (“Giant Proximity Effect”).2
In this talk, I will review our most recent experiments on such films and superlattices, including XRD, AFM, angle-resolved TOF-ISARS, high-resolution TEM, transport, resonant X-ray scattering, low-energy muon spin resonance, ultrafast photo-induced RHEED, and ultra-high magnetic field measurements. The results include an unambiguous demonstration of strong coupling of in-plane charge excitations to out-of-plane lattice vibrations3 and the discovery of interface HTS.4
- 1I. Bozovic et al., Phys. Rev. Lett. 89, 107001 (2002); P. Abbamonte et al., Science 297, 581 (2002).
- 2I. Bozovic et al., Nature 421, 873 (2003); Phys. Rev. Lett. 93, 157002 (2004)
- 3N. Gedik et al., Science 316, 425 (2007); Z. Radovic et al., Phys. Rev. B 77, 092508 (2008)
- 4A. Gozar et al., Nature 455, 782 (2008).
Warren Grice, "The entanglement your mother never warned you about"
In most photonic quantum information applications, information is encoded into the photons’ polarization degrees of freedom. This is a natural choice, given that polarization can be completely described by a linear combination of only two basis states. It has experimental appeal, as well, since it is relatively easy to manipulate polarization using simple optical elements. However, a more complete description of the photon also includes its energy and its spatial mode. And while it might seem that these have little to do with polarization, it turns out that spatial and spectral entanglement can have adverse effects in polarization entanglement experiments. The scientist who ignores these other degrees of freedom does so at his or her peril. I will discuss the source of these auxiliary entanglements and will present a series of theoretical and experimental results illustrating the subtle relationships between various types of entanglement. Strategies for mitigating unwanted effects will also be discussed.
Denis Dujmic, "Toward a dark matter telescope"
The nature of dark matter that makes 80% of the matter in the universe still remains a mystery. The dark matter may come in form of Weakly Interacting Massive Particles (WIMPs) and its detection will require a superb rejection of radiation backgrounds around and inside a detector. This alone, however, may not be enough to unambiguously identify the dark matter signal. A novel approach reconstructs direction of dark matter particles and allows correlation of the WIMP direction with the galactic motion through the dark matter halo. This talk will give an overview of WIMP searches and describe the directional approach by the DMTPC collaboration. Results from a prototype detector operated in a low-energy neutron beam, in particular, the first observation of the "head-tail" effect for low-energy nuclear recoils, clearly demonstrate the potential of our detector.