Physics Department
Fall 2004 Colloquium Schedule
Speaker: Prof. Carl Weinman, JILA and the University of Colorado Title: Using the tools of physics to teach physics
Speaker: Dr. Michael Cappiello, Los Alamos Title: Environmental Quality and Energy Security: A Global Vision for the Future of Nuclear Energy Abstract can be found here
Speaker: Prof. Dana Anderson, JILA and the University of Colorado Title: Problems at a Cocktail Party and Other Nonlinear Stories Abstract can be found here
Speaker: Prof. Joel Ullom, NIST Title: Superconducting Refrigerators and Sensor Arrays for Materials Analysis and Astronomy
Abstract can be found here
Speaker: Prof. Meredith Betterton, University of Colorado, Department of Applied Math Title: Structure formation on ablating surfaces: what do glaciers, silicon, and comets have in common? Abstract can be found here
Speaker: Prof. Chang Kee Jung, Stony Brook University Title: Einstein's Dream, Neutrino Revolution and UNO Abstract can be found here
Speaker: Prof. Kristi Anseth, Howard Hughes Medical Institute and the Department of Chemical Engineering, University of Colorado Title: Polymer Gels Designed for Cartilage Regeneration Abstract can be found here
Speaker: Prof. Jamie Nagle, University of Colorado Title: Six Microseconds After the Big Bang and Today: The Quark-Gluon Plasma
Abstract can be found here
Speaker: Prof. Eric Zimmerman, University of Colorado Title: Neutrino Mass and Flavor Mixing Abstract can be found here
Speaker: Prof. Harry Nelson, University of California, Santa Barbara Title: Listening for the Dark Abstract can be found here
Speaker: Prof. Sajeev John, University of Toronto Title: Photonic Band Gap Materials: Engineering the Fundamental Properties of Light Abstract can be found here
Speaker: Dr. Antoinette Taylor, Center for Integrated Nanotechnologies, Los Alamos National Laboratory Title: Ultrafast Dynamics in Complex Materials Abstract can be found here
Speaker: Dr. Andrew Wilson, University of Otago, New Zealand, and JILA. Title: A Collider for Ultracold Atoms - Imaging Partial-wave Interference in Quantum Scattering Abstract can be found here
Thanksgiving
Speaker: Prof. Matthew P. A. Fisher, Kavli Institute for Theoretical Physics, UC Santa Barbara Title: Quantum Choreography: Exotica inside Crystals Abstract can be found here
Speaker: Prof. Kent W. Staley, Saint Louis University Title: The Evidence for the Top Quark: Disputes and Statistics Abstract can be found here
Dr. Michael Cappiello, Los Alamos. September 1, 2004
Environmental Quality and Energy Security: A Global Vision for the Future of Nuclear Energy
Energy underpins the stability and prosperity of the nation and the world. The energy demand will rise substantially in the 21st century as nations, particularly those in the developing world, pursue affluence for the betterment of their populations. At the same time, such pursuits can lead to environmental insults and international tensions. To alleviate the pending environmental crisis, and the issues over energy and economic security, a diversified portfolio of energy sources, including nuclear, will best guarantee success. The positive attributes of nuclear energy are impressive, including emissions-free energy, security of fuel supply, the highest levels of industrial safety, and economic attractiveness of well-managed plants. On the other hand, the challenges of nuclear waste disposal and possible proliferation risks from nuclear materials continue to be dominant issues both domestically and internationally, and can stifle expansion. To better manage nuclear materials, a new international governance structure is proposed in which regional centers provide nuclear fuel services, all under strict international control and surveillance. Under the governance structure, implementation of advanced partitioning and transmutation technologies will minimize the waste needing long-term isolation in addition to minimizing the material diversion risk through the destruction of plutonium. Implementing this vision will require strong US leadership and a reinvigoration of the nuclear science and technology infrastructure. An international pilot project to demonstrate closed fuel-cycle and materials-management technologies will lay the groundwork for implementation.
Professor Dana Z. Anderson. September 8, 2004
Problems at a Cocktail Party and Other Nonlinear Stories
A listener in a crowded room of many voices can pay attention to one voice despite the interference of other speakers and sources of noise. This, the Cocktail Party Effect, is a popular example of what is commonly referred to as blind signal separation: given two or more signals that are scrambled one wants to descramble them without fore knowledge about how they are scrambled and, moreover, with little a priori information about the original signals in the first place. It is at first sight an impossible problem that can be stated this way: I’m not going to give you the matrix but I want you to give me its inverse anyway. To help you I will give you some result vectors of matrix-vector multiplications, but I won’t give you the original vectors.
The Cocktail Party Effect sets the stage for a discussion of physical dynamical systems that solve useful though difficult information processing problems. Well-behaved physical systems reach a steady state by extremizing an energy function; the key is to construct a physical system that solves a problem of interest as it extremizes its own energy. A ball rolling down a surface of a hill with friction can be said to solve the problem of finding a minimum of a particular function, however dull that specific problem might be. The cocktail party problem is more interesting and more difficult as well. We present a nonlinear holographic system whose potential energy function is determined by the statistics of the scrambled input signals themselves. As the system falls into a potential energy minimum, so it descrambles mixed signals. In their dynamical role here stochastic forces play practically the reverse role they normally do in physics.
Professor Joel Ullom. September 15, 2004Superconducting Refrigerators and Sensor Arrays for Materials Analysis and Astronomy
Superconducting sensors provide unmatched performance for certain applications in materials analysis and astronomy. For instance, transition-edge calorimeters provide a spectral resolving power in excess of 2000 at soft x-ray wavelengths. We describe recent design changes that made this performance possible. We also describe ongoing efforts to build arrays of 1,000 or more of these devices, for instance for the James Clerk Maxwell Telescope on Mauna Kea. Curiously, superconducting electronics can also be used to provide the 100 mK operating temperatures these sensors require. We describe recent progress with refrigerators based on normal-insulator-superconductor tunnel junctions. We have demonstrated devices with both technologically useful temperature reductions and cooling powers, and have now used them to cool bulk payloads.
Professor Meredith Betterton. September 22, 2004
Structure formation on ablating surfaces: what do glaciers, silicon, and comets have in common?
High-altitude snowfields exposed to intense sunlight become covered with spike-shaped ablation structures called penitentes. This talk will introduce field observations of ablation morphologies, including penitentes and suncups (which form on clean snow) and ablation hollows and dirt cones (which form on debris-covered snow). I will discuss a minimal model to describe formation of these structures. The dependence of ablation morphologies on weather conditions and initial dirt thickness will be described. I will compare the model predictions to recent laboratory experiments, which reproduced penitente-like structures in a controlled environment. Finally, the talk will describe other applications where these phenomena are important, in particular the formation of conical structures by laser ablation of materials and the surface morphology of comets.
Professor Chang Kee Jung. September 29, 2004
Einstein's Dream, Neutrino Revolution and UNO
In 1980's, large water Cherenkov detectors (IMB and Kamiokande) were constructed primarily to search for proton decays that were predicted by the Grand Unification Theories. Since then larger and more sensitive water Cherenkov detectors (Super-Kamiokande and SNO) were built and these detectors together with the earlier detectors have played the central role in the remarkable advancement made in neutrino physics during the last two decades. Their accomplishments include: Observation of neutrino oscillations (mass), Observation of neutrinos from a supernova (SN1987A) explosion, First real time and directional observation of solar neutrinos, Resolution of the solar neutrino problem, and Setting the world best limits on proton decays.
In order to continue to explore the physics at the Grand Unification Scale and even at the Plank Scale as well as
the rich neutrino physics we have just unveiled, we propose to build an even larger next generation water Cherenkov detector, UNO (Underground Nucleon decay and Neutrino Observatory).In this talk, I will discuss the physics potential and feasibility of the UNO detector along with a brief historical review of the proton decay searches and its connection to the neutrino oscillations. I will also introduce a newly discovered, excellent candidate site, the Henderson mine at Empire, Colorado, which is now proposed as a DUSEL (Deep Underground Science and Engineering Lab) candidate site. CU is a participating member in the Henderson Undeground Science and Engineering Project.
Professor Kristi S. Anseth. October 6, 2004
Polymer Gels Designed for Cartilage Regeneration
Our research group has been interested in the development and use of multifunctional monomers that can be photopolymerized to form crosslinked, degradable networks. Such materials provide advantages for numerous biological and medical applications, and when combined with photoinitiated polymerization, networks can be formed in situ under physiological conditions with temporal and spatial control of the polymerization process. In this area, our efforts have ranged from the design of multifunctional anhydride monomers that react to form highly crosslinked and surface eroding networks for fracture fixation applications to high molecular weight macromers that produce loosely crosslinked and bulk degrading hydrogels for cartilage tissue engineering. While the application of multifunctional monomers to produce degradable and crosslinked polymers is diverse and exciting, relationships between the macroscopic properties of the networks and their constantly changing microscopic characteristics are quite complex. In particular, this work focuses on experimentally characterizing and theoretically modeling the degradation behavior of networks synthesized from multifunctional poly(ethylene glycol) and poly(vinyl alcohol) based macromers. A statistical-kinetic model was developed to predict the influence of degradation, as well as various chemical, environmental, and processing parameters, on the macroscopic gel properties. Understanding the factors controlling the degradation of crosslinked gels allows networks to be synthesized that are specifically tailored for desired applications. To illustrate this concept, chondrocytes were encapsulated in these hydrogel matrices, and relationships between the network properties and the biochemical content and histological features of the resulting cartilaginous tissue investigated.
Professor Jamie Nagle. October 13, 2004
Six Microseconds After the Big Bang and Today: The Quark-Gluon Plasma
Quarks and gluons are the fundamental particles that interact via the strong (nuclear) force, and yet they are always confined inside hadrons. In the earliest stages of our Univerese the temperature was high enough that all the matter existed in a quark-gluon plasma where the quarks and gluons are not confined. We hope to re-create this plasma in the laboratory by colliding heavy nuclei at close to the speed of light. The PHENIX experiment is designed to study this plasma. Recent experimental results reveal some exciting new properties of this plasma.
Professor Eric Zimmerman. October 20, 2004
Neutrino Mass and Flavor Mixing
The last ten years have seen a revolution in our understanding of the properties of neutrinos. These neutral weakly-interacting particles, assumed to be massless in the minimal Standard Model of particle physics, have been shown to exhibit flavor "oscillations," a quantum mechanical interference effect that is only possible if neutrinos have mass and their flavor mixing matrix is not diagonal.
This talk will cover direct searches for neutrino mass as well as oscillation studies. The current data may be inconsistent with a simple mixing model. Current efforts at CU and elsewhere are underway to resolve this situation in the near term. The longer-term goal of a detailed understanding of the physics of neutrino flavor mixing (which may include CP violation) will require long-baseline oscillation experiments. In these, a neutrino beam is created at an accelerator laboratory, passes through the earth, and is detected several hundred kilometers distant. Several of these experiments are in the planning or construction stages.
Professor Harry Nelson. October 27, 2004
Listening for the Dark
One of the pressing scientific questions of our time is, what is the nature of the dark matter? The evidence for dark matter is overwhelming, but we know very little about its properties. There is an intense world-wide effort underway to directly detect dark matter in the laboratory. Much of this effort is directed toward detecting so-called WIMPs: neutral particles with a mass and an interaction cross section with our matter characteristic of the weak interaction. I'll discuss the approach that the `Cryogenic Dark Matter Search' or CDMS collaboration has taken to detect WIMPs, where we listen for the sound that WIMPs might make in germanium crystals at temperatures near absolute zero. Recently, our sensitivity to WIMP dark matter at our experiment at the Soudan Mine in northern Minnesota has become the best in the world.
Professor Sajeev John. November 3, 2004
Photonic Band Gap Materials: Engineering the Fundamental Properties of Light
Photonic Band Gap (PBG) materials are artificial, periodic, dielectrics that enable engineering of the most fundamental properties of electromagnetic waves. These properties include the laws of refraction, diffraction, and spontaneous emission of light. Unlike traditional semiconductors that rely on the propagation of electrons through an atomic lattice, PBG materials execute their novel functions through selective trapping or "localization of light" using engineered defects within the dielectric lattice. This is of great practical importance for all-optical communications and information processing. Three dimensional (3D) PBG materials offer a unique opportunity for simultaneously (i) synthesizing micron-scale 3D optical circuits that do not suffer from diffractive losses and (ii) engineering the electromagnetic vacuum density of states in this 3D optical micro-chip. This combined capability opens a new frontier in integrated optics as well as the basic science of radiation-matter interactions.
We review recent approaches to micro-fabrication of photonic crystals with a large 3D PBG centered near 1.5 microns. These include direct laser-writing techniques and holographic lithography. We review the concept of a hybrid 2D-3D PBG hetero-structure in which a 2D photonic crystal micro-chip layer is suitably lattice matched and embedded within a 3D PBG material. This microchip layer contains optical wave-guides and optical micro-cavities that enable frequency selective control of spontaneous emission of light from atoms. Unlike traditional wave-guides that confine light in a high refractive index medium using total internal reflection, these air-wave-guides operate using the principle of light localization for confinement of light along a low refractive index path.
We demonstrate a nearly universal approach to ultra-dense, three-dimensional, integrated optics in general 3D PBG architectures. These 3D optical circuit paths are constructed using broadband, loss-less, chip-to-chip interconnects between 2D micro-chip layers, intercalated within the 3D PBG host material. Unlike electronic micro-circuitry, each air-wave-guide path can simultaneously conduct hundreds of wavelength channels of information, throughout the 3D micro-chip.
In addition to exhibiting diffraction-less flow of light through micron-scale bends, this optical micro-chip allows the engineering of very large and abrupt changes in the local electromagnetic density of states as a function of frequency. This leads to unprecedented frequency selective control of spontaneous emission, modification of the blackbody radiation spectrum, and some fundamentally new optical functions unattainable in conventional photonics.
Dr. Antoinette J. Taylor. November 10, 2004
Ultrafast Dynamics in Complex Materials
I will discuss the development and application of novel optical spectroscopic techniques to the study of ultrafast dynamics in complex materials. I will first describe all-optical pump probe and optical-pump far-infrared probe experiments on (a) colossal magnetoresistance manganites, (b) superconductors, and (c) heavy fermion materials. The experimental techniques are discussed followed by a brief review of ultrafast electron dynamics in conventional wide band metals that serves as a starting point in understanding dynamics in more complex systems. In. half-metallic manganites, the quasiparticle dynamics in the ferromagnetic metallic state can be understood in terms of a dynamic transfer of the spectral weight which is influenced by the lattice and spin degrees of freedom. For high temperature superconductors, ultrafast quasiparticle dynamics are sensitive to the order parameter and superconducting pair recovery occurs on a picosecond timescale. Heavy fermion compounds reveal an anomalous slowing down of quasiparticle dynamics below the Kondo temperature. These results show that, in general, ultrafast optical spectroscopy provides a sensitive method to probe the dynamics of quasiparticles at the Fermi level. We have further extended these measurements to reveal dynamics in structured materials. In particular, we have developed a diagnostic that combines the nanoscale spatial resolution of an STM with ultrafast temporal resolution. This technique has been used to study electron and hole dynamics following photoexcitation in InAs self-assembled quantum dots. Finally, the interaction of ultrafast optical pulses and microstructured fibers is visualized and studied using novel techniques that simultaneously display the spectral and temporal characteristics of a pulse. These techniques are used to investigate the nonlinear dynamics associated with the interaction between dispersive waves and solitons in the vicinity of the second zero dispersion point of photonic crystal fibers.
Dr. Andrew Wilson. November 17, 2004
A Collider for Ultracold Atoms - Imaging Partial-wave Interference in Quantum Scattering
We have directly imaged s and d partial-wave interference in the cold collision of atoms. A double-well magnetic trap was used to produce two ultracold (200 nK) clouds of 87Rb atoms. Collisions at a selectable energy (up to 1 mK) occur when the trapping potential is continuously modified to a single-well configuration. The atomic clouds accelerate from the sides of the harmonic potential and collide at the center of the well. The resulting scattering is equivalent to cold collisions of counter-propagating ultracold pulsed atomic beams. Angular resolved detection of scattered atoms is obtained using laser absorption imaging. As a consequence of a d-wave shape resonance in the collision cross-section, we observe scattering patterns evolving from s-wave-like to d-wave-like, via s+d interfering scattering states, which expose the quantum mechanical origin of the collision process. Since only two partial wave states are involved, the scattering patterns are simple to interpret.
Professor Matthew P. A. Fisher. December 1, 2004
Quantum Choreograpy: Exotica inside Crystals
The electrons moving through ordinary solids such as a piece of Iron can sometimes self-organize in novel ways. Due to their fuzzy quantum motion, this can lead to very unusual (and useful!) behavior, as with the resistanceless flow of electricity in superconductors. But even more exotic patterns of quantum behavior are possible, where the electron is effectively "splintered" into distinct fragments. Searching for such exotic "quantum dancing patterns" involves trying to intuit the quantum choreography that nature employs. I will describe the efforts and prospects in this exciting search.
Professor Kent W. Staley. December 8, 2004
The Evidence for the Top Quark: Disputes and Statistics
This talk centers on the CDF Collaboration's 1994 paper presenting the first evidence for the existence of the top quark. I sketch the various strategies developed within CDF for identifying top quark decay events, and the disagreements within the collaboration over those strategies. In particular I examine how suspicions arose within the collaboration that various components of the top search were plagued by biases, as well as how and to what extent those suspicions were resolved. The history of this result exemplifies how weaknesses at the organizational and personal level in a collaboration can lead to difficulties in making a persuasive argument, while also highlighting the power of new data and the reanalysis of old data as a means of revealing and remedying bias.