Spring 2007 Physics Colloquium Schedule

University of Colorado Department of Physics

Colloquia are Wednesdays at 4 PM, Duane G-1B20 unless otherwise noted

Coffee, tea, and cookies before regular colloquia at 3:45 PM in Duane G-1B31

Upcoming Colloquia:

Colloquia that have already occurred:

• January 17
  • Sean Carroll, Caltech
  • Host: Oliver DeWolfe
  • Title: Dark Energy, or Worse?
  • Abstract: Our universe is accelerating and we don't know why. Together, ordinary matter and ordinary gravity are unable to account for this phenomenon. The simplest explanation invokes "dark energy," a smooth and persistent component of the universe's energy budget, which could be a cosmological constant or a slowly-varying field. Alternatively, Einstein's theory of general relativity could be breaking down on cosmological scales. I will discuss the basic evidence for the accelerating universe, some of the theoretical proposals that have been put forward to account for it, and future observational tests that will help us distinguish between the possibilities.
• January 24 COLLOQUIUM CANCELED DUE TO ILLNESS.
• January 31 Physics Colloquium lends G-1B20 to the Mathematics Department's 43rd Annual DeLong Lecture Series (please attend)
  • Roger Penrose, Oxford University and Penn State University
  • Host: Richard Green
  • Title: Twistor Theory: Old and New
  • Abstract: Basic twistor geometry and its description of massless free fields. Twistor theory revives some 19th-century geometry of Sophus Lie and Felix Klein, and puts it to use to re-express and generalize elegant representations (some put forward at the turn of the 20th century) of some of the most basic fields of physics. The theory indicates a possibly deep role for holomorphic concepts (complex manifold structure, holomorphic sheaf cohomology, complex bundles) in the key equations of physics, relating them to foundational aspects of quantum theory. Some recent developments will be outlined, such as the twistor-string theory promoted by Edward Witten and a novel approach to the resolution of divergences in quantum field theory due to Andrew Hodges.
  • Dates, Times, Locations and Abstracts for Roger Penrose's 1st and 3rd DeLong Lectures can be found here
• February 7
  • Albert A. Bartlett, University of Colorado
  • Host: John Cumalat
  • Title: Sustainability 101: Arithmetic, Population and Energy
  • Abstract: This talk examines the mathematical and practical consequences of steady growth or quantities such as populations and rates of consumption of resources. It is disturbing in the extreme that public officials argue for the continuation of growth of populations and of the economy when it is obvious that this growth violates the First Law of Sustainability; "Population growth and / or growth in the rates of consumption of resources cannot be sustained."
• February 14
  • Alberto Fernandez-Nieves, Harvard University
  • Host: Noel Clark
  • Title: Nematic order in spherical shells
  • Abstract: We use double emulsion drops to experimentally realize a system to investigate the defect structure in spherical shells of nematic liquid crystal. The ground state of this system is predicted to exhibit a tetrahedral arrangement of four surface defects in a structure reminiscent of a baseball. Instead, we find a much richer set of coexisting defect structures dictated by the inevitable thickness of even the thinnest shells, and the resultant variation of the shell thickness around the sphere. These structures are characterized by a varying number of disclination lines and pairs of surface point defects, one each on the inner and outer surfaces of the nematic shell. The inevitable thickness variation of the shell determines the separation of the defects, and the ultimate configuration. In the limit of thick shells, the structure ultimately merges with that of a bulk nematic liquid crystal drop.
• February 21
  • Kiyong Kim, Los Alamos National Lab
  • Host: Scott Robertson
  • Title: High-intensity laser-plasma interactions: ultrafast X-ray and terahertz spectroscopy
  • Abstract: The interaction of high-intensity, ultrashort laser pulses with matter is of great current interest because such experiments can access the extreme conditions of hot dense plasmas previously seen only in either astrophysical or particle accelerator settings: gigabar pressures, megavolt temperatures, relativistic particle dynamics, and nuclear reactions. Potential applications include intense short pulse X-ray generation, attosecond physics, plasma-based particle acceleration, and inertial confinement fusion using a fast-ignition concept. In these studies, spectroscopic diagnostics of photon's energy ranging from X-ray to terahertz (THz) play a vital role in uncovering the ultrafast dynamics. Using optical and X-ray diagnostics, we have investigated femtosecond time-resolved dynamics in the interaction of intense laser pulses with nanometer-size clusters, revealing the details of explosion dynamics of laser-heated nanoplasmas. From X-ray spectroscopy of centimeter-long plasma channels, produced by self-guided pulse propagation in elongated cluster jets, we have observed Doppler blueshifted X-ray emission from fast ions of exploding clusters; truncated redshifted emission from the opposite side of the cluster. In two-color laser-air interactions, we have observed coherent THz electromagnetic pulse emission from a plasma current surge in symmetry-broken laser fields. Our understanding of THz generation mechanism will speed further optimization and scalability of tabletop, ultrafast THz sources.
• February 22 - Note special date and place: Thursday at 4 PM, JILA Auditorium
  • Jan Lagerwall, University of Stuttgart
  • Host: Noel Clark
  • Title: Anisotropic fluids hosting anisotropic nanoparticles: Carbon nanotube-liquid crystal composites
  • Abstract: Liquid crystals (LCs) constitute a fascinating and rich field of research within soft matter physics and chemistry. While these anisotropic fluids are perhaps best known for their use in displays (LCDs), current academic research is largely focusing on other issues. The two main properties of LCs which are in the research spotlight today are their huge response function to external influences (e.g. electric or magnetic fields, temperature changes or chirality)-rendering LCs interesting for use in sensing devices, modulators and actuators-and their inherent ability to self-organize with orientational and / or translational order of varying degree and dimensionality. The characteristic length scales go from the nano- to the micrometer range and at least in the case of nematic LCs (exhibiting orientational but no translational order) a high degree of fluidity is maintained. The resulting structures are currently being employed for instance as templates for nanoporous silica or in the creation of complex composites with new functionalities, including colloidal systems where the LC can constitute either the solvent or the interior of the particles. In this talk I will, after giving a general introduction to the liquid crystalline state of matter and to modern LC research, concentrate on the employment of LC self-organization, giving examples from my current main research project: the preparation and study of carbon nanotube-liquid crystal composites1. During the last few years a number of stimulating reports have revealed mixtures of carbon nanotubes (CNTs) in LCs to be multifaceted and attractive composites. On the one hand they have application potential in diverse areas of science and technology, on the other the meeting between CNTs and LCs also brings up many new intriguing questions of fundamental research character. After having shown (using polarized resonant Raman spectroscopy) that CNTs align along the LC director in lyotropic as well as thermotropic LCs my collaborators and I are now trying to better understand the sensitive stability of the CNT-LC dispersions. Both types of LC can disperse the CNTs well, but incorrect handling can quickly induce nanotube flocculation. It turns out that not only the direct intermolecular interactions between the CNT guest and the LC host, but also the elastic interactions within the composite medium as a continuum, must be considered to understand the behavior of these systems. We also look into the effects that the CNTs may have on the LC, in particular regarding phase sequence, switching dynamics and electrical properties.
• February 28
  • Michi Nakata, Giuliano Zanchetta, Tommaso Bellini, and Noel Clark, University of Colorado
  • Host: Chuck Rogers
  • Title: Liquid Crystals and the Origin of Life: Michi's Story
  • Abstract: Scenarios for the appearance of life on earth divide time into pre-biotic and metabolic eras, the latter commencing with the active harvesting of energy. The ubiquity of RNA in the key processes of life leads to the prevailing opinion that RNA or something like it preceded metabolism, appearing in the prebiotic era in the form familiar from DNA and RNA: pairs of linear polymers consisting of water soluble chains stitched together as a pair by the complementary hydrogen bonding of aromatic hydrocarbon side groups (base pairs). The "RNA world", transitioning between the pre-biotic and metabolic eras, required such polymers complex enough to be selectively replicated and to exhibit catalytic activity, something like at least 35 base pairs in length. One of the prevailing mysteries is how, with only energy input and dynamic environmental conditions, such complex molecules could appear out of the "gemisch", the complex mixture of aromatic hydrocarbons, sugars, phosphates, and other inorganics believed to be available in aqueous environments of the prebiotic earth. Clearly, their organizing principle had nothing to do with biology. We will present a new scenario for this development, based on our recent observations of unexpected liquid crystal phases in nanoscale DNA, short DNAs 6 to 20 base pairs in length. The wisdom garnered over the past 50 years or so is that life's information carriers, DNA and RNA, form liquid crystal phases because they are rod-shaped semiflexible polymers. We will argue that it is actually the other way around - life's information carriers are rod shaped polymers because such molecules form liquid crystal phases.
• March 2 - Note special date and place: Friday at 4 PM, G-125
  • Lars Stixrude, University of Michigan
  • Host: Mike Ritzwoller
  • Title: Silicate liquids in planetary interiors
  • Abstract: Silicate liquids are primary agents of mass and heat transport in terrestrial planets, yet little is known of their physical properties or structure over most of the relevant pressure regime. We have applied density functional theory to the study of silicate liquids via Born-Oppenheimer first principles molecular dynamics. We explore the variation of liquid structure, electronic structure, density, dynamics, and phase stability (freezing) over the terrestrial pressure-temperature regime for a variety of geophysically important compositions in the MgO-CaO-H2O-SiO2 system. We draw conclusions regarding the presence and buoyancy of silicate melt at depths greater than the typical magma genetic regime, including the base of Earth's mantle, and implications for terrestrial chemical and thermal evolution.
• March 7
  • Scott Hsu, Los Alamos National Laboratory
  • Host: Scott Robertson
  • Title: Magnetic relaxation in plasmas
  • Abstract: A turbulent magnetized plasma tends to evolve toward preferred equilibrium states. Known as "magnetic relaxation," this intriguing process occurs in space/astrophysical plasmas and can potentially be harnessed to generate plasma configurations for eventual use in a fusion reactor. In this colloquium, I will give an introduction to the plasma physics of magnetic relaxation and then present experimental results relating to two underlying mechanisms that facilitate magnetic relaxation: magnetic reconnection and magnetic helicity injection. I will conclude the talk by describing exciting recent advances in magnetic relaxation theory, which have suggested new research directions in this area, and a new experiment coming online at Los Alamos, the Driven Relaxation Experiment, designed to explore the physics of magnetic relaxation in driven systems and the possibility of forming novel relaxed plasmas. The new experiments can potentially establish the physics basis for designing novel compact fusion plasma configurations and for solving other key problems in the fusion program, e.g., non-inductive plasma startup in a spherical tokamak.
• March 12 - Note special date and place: Monday at 4 PM, G-125
  • Dan Needleman, Harvard University
  • Host: Noel Clark
  • Title: Single Molecule Dynamics in Cell Division
  • Abstract: A wide variety of subcellular structures exist in a non-equilibrium steady state with a constant flux of molecules and energy continuously modifying and maintaining their architecture. A prime example of this is the spindle: a remarkable, self-organizing molecular machine that segregates chromosomes during cell division. The spindle is a highly dynamic structure composed of the protein tubulin, which assembles into long polymers called microtubules, and a variety of other proteins that regulate microtubule nucleation, polymerization, depolymerization, and translocation. We investigated the motions of thousands of individual tubulin molecules in spindles, using a combination of single molecule confocal microscopy and automated particle tracking. This study allowed us to perform the first detailed analysis of microtubule polymerization in spindles. Our single molecule data can be quantitatively explained by a first-passage analysis of a very simple model of microtubule dynamics. These results unambiguously rule out a number of proposed mechanisms of microtubule turnover, leading to surprising implications for models of spindle organization.
• March 14
  • Irene Dujovne, Delft University of Technology
  • Host: Noel Clark
  • Title: Optical tracking of molecular processes at high resolution with a novel traveling wave technique
  • Abstract: In order to understand the basic mechanisms of living cells, it is essential to elucidate the complexity of fundamental interactions and dynamical processes between their underlying molecular building blocks, such as DNA, RNA, and proteins. High resolution, non invasive, optical methods are outstanding tools that can provide us with unique insights into such sophisticated molecular machineries. In this talk I will present a new optical experimental technique which is highly sensitive and minimally invasive that allows, in particular, study of real time dynamics of molecular motors in the single molecule limit. These dynamical processes are essential to many cell functions, such as intracellular transport, cell division, growth, and locomotion. The extremely high temporal (~10 microsecond) and spatial (subnanometer) resolutions provided by the new method may be key to studying various interactions and dynamical mechanisms of molecular motors. I will describe the basic physical principles of the technique, and will discuss preliminary results from several systems, whose detailed study is currently underway.
• March 19 - Note special date and place: Monday at 4 PM, G-125
  • Ivan Smalyukh, University of Illinois at Urbana-Champaign
  • Host: Noel Clark
  • Title: Probing and Controlling Order in Soft Matter
  • Abstract: Future technologies and devices require materials with well-controlled periodic structures in the nanometer and micrometer ranges, as well as with properties such as optical anisotropy and mechanical flexibility. Ordered soft materials can meet all of these requirements, can respond to low external fields, and can be manipulated by light. This makes them ideal for tunable photonic and all-optical applications, which, however, critically depend on finding the means to control the molecullar and colloidal self-organization as well as on the knowledge of the underpinning physical phenomena. This lecture will discuss optical non-contact control and three-dimensional imaging of self-organized structures in anisotropic soft materials such as liquid crystals and biopolymers. The focused laser beams can generate polarization-controlled optical forces in these media. Unlike in isotropic fluids, colloidal particles can be set to follow well-controlled trajectories solely by changing the polarization state of a stationary-focused laser beam. This polarization-controlled motion of a colloidal particle into or away from the focused beam can be navigated over distances much larger than the particle's radius, because of the "corona" of well-defined molecular alignment and of the effective refractive index around the bead. Polarized-state optical manipulation is possible even in single-component materials with homogeneous chemical composition but spatially-varying molecular orientations and is applied to defects and self-organized structures. Moreover, when the laser beam's intensity exceeds the threshold for the optically-induced realignment, a variety of localized structures can be first optically generated and then spatially translated and organized into superstructures such as periodic arrays. To visualize the three-dimensional patterns of molecular orientations, the fluorescence confocal imaging is performed with a well-controlled polarization states of probing light as well as the used dye molecules follow orientations of the material's host molecules. Optical imaging and trapping techniques provide insights into the physical mechanisms behind the self-organized periodic structures in materials ranging from liquid crystals to DNA biopolymers. The optically-directed periodic structures have a potential for applications in all-optical devices, tunable photonic crystals, and for optical data storage.
• March 21
  • Yael Roichman, New York University
  • Host: Noel Clark
  • Title: Lights, camera, action: Nonequilibrium processes in holographically driven colloid
  • Abstract: Colloidal dispersions of micrometer-scale spheres have been widely exploited as model system for fundamental studies in statistical physics. Small enough to undergo random Brownian motion, yet large enough to see with a conventional light microscope, these particles readily form equilibrium many-body systems whose microscopic degrees of freedom can be measured directly with digital video microscopy. By applying precisely structured forces to individual spheres with computer-generated holograms, we are able to extend these studies to non-equilibrium many-body and statistical processes.Here we will demonstrate the various ways by which holographic optical tweezers can contribute to these studies both by using the conventional trap arrays and by innovative continuous traps designed by the shape-phase holography technique.
• March 28
  • Spring Break, no Colloquium
• April 4
  • Oren Cohen, University of Colorado
  • Host: Scott Robertson
  • Title: Nonlinear x-ray photonics
  • Abstract: Ever since the invention of the laser, scientists have been striving to extend coherent beams into the x-ray region of the spectrum. High harmonic generation (HHG) Ð coherent upconversion of intense visible or IR laser light into extreme ultraviolet and x-ray radiation Ð is considered a promising process for accomplishing this goal in a tabletop system. However, HHG is associated with ionization of atoms in a gas which leads to plasma, poor phase matching conditions, inefficient conversion, and low flux that limits applications. In this talk, I will present new approaches that use crystals made of light for manipulating the emitted harmonics and phase matched the generation process, hence, significantly increasing the conversion efficiency. For example, enhancement factor larger than 300 was demonstrated for harmonics at 70 eV by a train of weak counter-propagating pulses. The optically induced structures can be used for controlling the spectral, transversal, and temporal properties of the generated harmonics beam. Finally, I will show that applying these techniques for generating high harmonics from ions in waveguides made of plasma is very promising for obtaining a bright tabletop laser-like hard x-ray radiation that will open new directions in bio-, nano-, materials-, and molecular probing, on ultra-fast timescale.
• April 5 Joint APS/JILA/Physics Department Colloquium
  • Note special date and place: Thursday at Noon, JILA Auditorium
  • Robert Kirshner, Harvard University
  • Host: Dick McCray
  • Title: Fundamentals of Supernova Cosmology
  • Abstract: Supernova observations form the foundation of evidence for an accelerating universe. Diligent work on supernova spectroscopy and supernova photometry in the nearby Universe allows for tests of some of the assumptions that make supernovae so powerful at high redshift. They also provide unique probes of the local dark matter distribution through measurement of local flows. The current state of the CfA program at low-z, the ESSENCE program at intermediate redshift, and the Higher-Z program with the HST will be discussed, and the best inferences for the properties of dark energy presented. A flat universe with a cosmological constant for the dark energy is consistent with all the current constraints.
• April 6 - Note special date and place: Friday at 4 PM, G-1B30
  • Gianluca Gregori, Rutherford Appleton Laboratory
  • Host: Scott Robertson
  • Title: High Energy Density Physics and Dynamics of Strongly Correlated Plasmas
  • Abstract: Dense plasmas and warm dense matter (WDM) in general encompass a unique state of matter where the system exhibit both gas-like and solid-like properties. WDM is found in many astrophysical enviroments (white dwarfs, neutron star atmospheres and interior of planets) and, in the laboratory, in laser produced plasmas and during inertial confinement fusion (ICF) experiments. WDM poses formidable theoretical challenges as neither density functional and path integral approaches nor plasma expansion techniques are directly applicable. In this talk we will give a general introduction to WDM, and high energy density physics in general, with application to ICF and laboratory astrophysics. In the past, diagnostics methods have been hindered by difficulties in producing high energy penetrating probes while emission spectroscopy techniques had often to rely on surface measurements. We will explore a novel diagnostics technique that has emerged in the last few years based on x-ray scattering, as an extension of the usual optical Thomson scattering used in low density plasmas. The various component of the scattering theory will be discussed in details. We will show that x-ray scattering can be implemented to both measure single particle dynamics and collective response (i.e., plasmons) in WDM, and used as a tool to validate dense matter theories. Example of measurements taken on laser facilities will be presented, with particular regard to the understanding of the microscopic response of strongly coupled plasmas. Future applications and extension of the technique using free electron lasers and ultra-fast electron sources will be briefly discussed.
• April 11
  • Edward Redish, University of Maryland
  • Host: Noah Finkelstein
  • Title: Problem Solving and the Use of Math in Physics Courses
  • Abstract: Mathematics is an essential element of physics problem solving, but as professionals, we often fail to appreciate exactly what we are doing with it. Math may be the language of science, but math-in-physics is a distinct dialect of that language that requires both more subtlety and more skills than are typically taught in math courses. Research with students in classes ranging from algebra-based physics to graduate quantum mechanics indicates that (1) we sometimes don't appreciate the skills students need to solve the problems we assign, and (2) students problems are sometimes with their expectations about what they are supposed to be doing rather than with their math skills. Implications for instruction in upper division physics classes will be discussed.
• April 18
  • Vera Luth, Stanford Linear Accelerator Center
  • Host: Steve Wagner
  • Title: What Can We Learn from Beta-Decays of Heavy Flavor Mesons?
  • Abstract: Historically beta decays have taught us a lot about fundamental properties of weak interactions, most importantly, they gave us evidence for neutrinos and parity violation. At present, semi-leptonic and leptonic decays of strange, charm and beauty mesons are of interest because they allow us to study the coupling to the charged weak current to quarks and also test our understanding of hadronic interactions of quarks bound in mesonic states. Recent experimental results will be reviewed and compared to theoretical predictions.
• April 19 - Note special date and place: Thursday at 4 PM, JILA Auditorium
  • Bernard Yurke, Bell Laboratories
  • Host: Dana Anderson
  • Title: Powering the nanoworld with DNA
  • Abstract: Two strands of DNA will bind most strongly to each other if their base sequences are complementary. The specificity of this molecular recognition property makes DNA a versatile material for use in the assembly of complex nanostructures. The free energy of hybridization, released as two DNA strands come together to form double-stranded DNA, can be used to induce nanostructures to perform mechanical work. Strand displacement allows one to operate such structures as cyclic engines. DNA can thus serve both as a structural material and as a fuel to power the nanoworld.
• April 25
  • Guoqiang Li, University of Arizona
  • Host: Joe Maclennan
  • Title: Diffractive Liquid Crystal Lenses for Vision Correction
  • Abstract: Many people have difficulty in focusing close objects onto the retina because of a decrease in accommodation with age, a condition called presbyopia. Conventional bifocal or trifocal lenses for this correction have been around for about 200 years, but they have limited field of view. An electro-optic lens allows voltage controlled change of the focusing power across the entire aperture. Such a lens must have high light efficiency, relatively large aperture, fast switching time, low driving voltage, and power-failure-safe configuration. These requirements have not been met simultaneously in the past. New switchable, flat, thin liquid crystal diffractive lenses that satisfy the above requirements will be discussed. The effects of the gaps between the ring electrodes and the fringing field on the lens performance have been analyzed. Design, fabrication, and characterization of three lenses will be presented. They provide the capability of corrections for near-, intermediate-, and distance-vision. Such a lens will have big potential impact on the field of vision care.
• May 2
  • Gary Staebler, General Atomics
  • Host: K.T. Mahanthappa
  • Title: Fusion Energy Research on Advanced Tokamak Physics
  • Abstract: World wide fusion energy research is entering a new phase with the November 2006 decision to construct ITER (International Thermonuclear Experimental Reactor) in Cararache France. This machine will be the first proof of principle experiment demonstrating a controlled self-heated fusion reaction with fusion power greater than the power needed to run the device. This is an important milestone on the way to a fusion power plant. In this talk the proposed development paths to comercial fusion power will be discussed including a fast track approach. A survey of some of the physics that has made it possible to build ITER with confidence in its success will be given. The ITER device is designed to run as a pulsed inductive "conventional" tokamak. The concept of an "advanced" tokamak is based on breakthroughs in the last two decades that have shown that significant improvement in the performance of a tokamak can be achived with active controls. Progress towards a steady state non-inductive, high pressure and high confinement advanced tokamak operation regime with active control of the internal plasma profiles will be illustrated. Some of the phenomena that challenge our understanding of the physics of a reactor grade plasma will also be presented.

Colloquium schedules from previous semesters can be found here

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