Fall 2010 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
- November 24
- Fall Break, no Colloquium
- November 29 Note special date and place: Monday at 4 PM,
- Paul Romatschke, Frankfurt Institute for Advanced Studies
- Host: Jamie Nagle
- Title: Looking inside neutron stars: Microscopic calculations confront observations
- Abstract: What are the thermodynamic properties of the matter found in the center of a neutron star? Despite several decades of research in nuclear physics and a wealth of astrophysical data, this seemingly simple question has yet to be answered. In this talk I will review state-of-the art neutron star observations and compare them to results derived from state-of-the-art microscopic calculations. A tentative answer to the question above will be given, and I will end this talk by describing future routes to constraining the equation of state for cold, dense matter.
- December 1
- Dam Thanh Son, University of Washington
- Host: Michael Hermele
- Title: Viscosity, quark gluon plasma, and string theory
- Abstract: 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 compute the viscosity for a class of strongly interacting fluids not too dissimilar to the quark gluon plasma. I will describe efforts to measure the viscosity and other physical properties of the quark gluon plasma at the Relativistic Heavy Ion Collider.
- December 2 Note special date and place: Thursday at 4 PM, JILA Auditorium.
- Christopher Lee, MIT
- Host: Jamie Nagle
- Title: Jets as Probes of the Fundamental Forces of Nature
- Abstract: Jets are treasure troves of information about the fundamental interactions among elementary particles. They are formed by energetic strongly-interacting particles decaying and showering into quarks and gluons which then bind into hadrons, all traveling in the same direction. Historically jets established fundamental properties of Quantum Chromodynamics (QCD) and even now contribute to the most precise extractions of the strong coupling constant. Today, especially with the advent of Large Hadron Collider (LHC), jets are important parts of signals (and backgrounds) for new particles beyond the Standard Model that decay into quarks and gluons, and also as probes of exotic phases of dense, strongly-interacting matter such as the quark-gluon plasma. I will describe these applications and also recent theoretical efforts to predict jet properties precisely using the tools of effective field theory. These include predictions for jet shapes, which characterize the internal structure of jets and contain clues about their underlying origin, information crucial in distinguishing new particles from backgrounds. Jet shapes can also reveal properties of new phases of dense QCD matter through which they pass and that we expect will be produced at the LHC.
- December 6 Note special date and place: Monday at 4 PM,
- Harvey Meyer, Johannes Gutenberg University, Mainz
- Host: Jamie Nagle
- Title: Extracting Real-Time Quantities from Euclidean Field Theory
- Abstract: In Quantum Chromodynamics, non-perturbative observables
such as the low-lying spectrum of hadrons can be extracted from the
theory discretized on a Euclidean space-time lattice and simulated on
a computer. Real-time quantities on the other hand, such as phase
shifts, resonance widths, or transport properties at finite
temperature, cannot be straightforwardly computed. In some cases
however real-time effects leave signatures in the Euclidean theory
which can be isolated in certain kinematical regimes. I describe
several examples of this type at zero and finite-temperature.
- December 8
- December 13 Note special date and place: Monday at 4 PM, JILA Auditorium.
- Abhijit Majumder, Ohio State University
- Host: Jamie Nagle
- Title: Probing QCD matter at the sub-femtometer scale
- Abstract: In the last several years, a lot has been learnt about the structure of hot QCD matter from heavy-ion collisions and lattice-QCD simulations. However, fundamental questions remain, such as what is the microscopic structure of the matter at temperatures not too far above the deconfinement transition temperature. While lattice-QCD has yielded some clues regarding the quark structure, the gluonic sector remains a mystery. The only known means of probing this structure is via the medium modification of hard jets. Recently, it was shown that one may describe the propagation of jets through dense matter using a transport-like setup. The transport coefficients, extracted by comparison with experiment, will reveal vital details of the structure of the medium. This theoretical setup is universal and can be applied to the study of both hot deconfined matter at RHIC and LHC, and cold nuclear matter in large nuclei at HERA and the future Electron Ion Collider (EIC). I will describe the major challenges of mounting such a theoretical framework, the progress that has been made so far, and its promise for a quantitative understanding of the structure of QCD matter.
Colloquia that have already occurred:
- August 25
- Michael Shull & Mitchell Begelman, University of Colorado
- Host: Dmitri Uzdensky
- Title: New Worlds, New Horizons in Astronomy & Astrophysics
- Abstract: The Astro2010 report, New Worlds, New Horizons in Astronomy and Astrophysics, was publically released in pre-publication form on August 13, 2010, via the National Academies Press website and an eTownHall webcast. In this colloquium, we will summarize the key
science, technology development, and prioritized missions and facilities recommended to
NASA, NSF, and DOE for the coming decade. The broad science objectives include:
searching for the first stars, galaxies, and black holes; seeking habitable planets; and
advancing our understanding of the fundamental physics of the universe. In the Astro2010
recommended plan are new optical/infrared survey telescopes (ground and space) to
investigate the nature of dark energy, determine the architectures of planetary systems,
observe supernova explosions, and open a window on the time-variable universe.
Ground-based telescopes include CCAT (a 25m sub-millimeter wave telescope in Chile),
LSST (large synoptic survey telescope), and GSMT (giant segmented mirror telescope).
Space missions include WFIRST (wide-field infrared survey telescope), LISA (laser interferometer
space antenna), and IXO (international x-ray observatory). The committee also recommended
enhancement of NASA Explorers, a New Worlds Technology Development (NASA), a
Mid-Scale Innovations Program (NSF), and support for individual and group programs in Astrophysical and Computational Theory.
- September 1
- Matteo Pasquali, Rice University
- Host: Ivan Smalyukh
- Title: Carbon nanotubes fluids: simple or complex?
- Abstract: At the single-molecule level, Single-Walled Carbon Nanotubes (SWNTs) have remarkable electrical and mechanical properties, more so than previously known polymer molecules or colloidal particles. Realizing these properties in applications requires understanding and controlling the behavior of SWNTs in fluid phases. Biological and environmental applications as well as sensing are likely to require dilute phases of SWNTs; material processing-for example for making coatings and fibers-will require more concentrated phases.
To some extent, SWNT liquids were (and still are) considered an oxymoron because dispersing or dissolving SWNTs into fluid phases proved exceedingly difficult. From the fundamental physics viewpoint, SWNT fluids should be complex, because of the presence of intermediate length scales between those of the solvent molecules and those of a macroscopic flow.
In this lecture, I will discuss how SWNTs can and should be viewed as hybrids between polymer molecules and colloidal particles. Even at low concentrations (few parts per million), SWNTs form complex fluid phases with intriguing properties. When stabilized properly, dilute SWNTs behave as Brownian rods. Indeed, we show that they are the best model system for studying the dynamics of stiff filaments, an important area of polymer physics and biophysics. By using SWNTs, we resolve a three-decade old controversy on the Brownian diffusion of slender rods in crowded environments. We also show that, when the solvent is appropriate and the concentration sufficiently high, SWNTs self-assemble into novel liquid crystalline phases. Understanding these phases requires extending classical theories to include the effects of short-range repulsion and long-range attraction between rods. We show that SWNT liquid crystals have great potential application in the manufacturing of well-aligned SWNT fibers.
- September 8
- Angelo Mascarenhas, NREL
- Host: Dan Dessau
- Title: Antisymmetry and the breakdown of Bloch’s theorem for light
- Abstract: Nature abounds in repetition: spatial, temporal, or spatio-temporal, and in space this visual rhythm is best orchestrated in the periodic atomic arrangements in crystalline lattices. It is from the collective behavior of such large many particle systems that the diverse physical properties of solids emerge, hence this subject (the study of the collective behavior of matter) lies at the core of condensed matter physics. In analyzing the physical behavior of systems exhibiting such periodicity, enormous simplification is afforded through the use of Bloch’s Theorem. The machinery developed for mathematically dealing with symmetry is based on Group Theory, and within this framework, Bloch’s Theorem emerges as a result of a physical system being invariant under the group of discrete translations. For almost a century, this theorem has become a tenet for dealing with the physics of systems that exhibit periodic translational symmetry. The present study utilizes the propagation of light in 2-D antisymmetric lattices to explore the physical consequences of this geometrical invariance. Using both classical and wave mechanical approaches, we show that despite their translational symmetry, Bloch’s Theorem is invalid for these lattices. Employing topological tools used in the mathematical treatment of billiard dynamics, we demonstrate that it is necessary to examine dynamical behavior in phase-space (as opposed to only in real-space) in order to account for the breakdown of Bloch’s theorem in these 2-D antisymmetric lattices. The unusual explanation underlying these surprising results implies that our intuitive notions of the physical consequences of invariance in space need to be reexamined. As such, the work has implications to almost every field where symmetry principles underlie the formulation of physical laws.
- September 15
- Tobin Munsat, University of Colorado
- Title: Imaging Techniques in Magnetically Confined Plasmas
- Abstract: It has long been recognized that plasma confinement in magnetic systems is limited by magnetohydrodynamic instabilities and small-scale turbulence, which can degrade or destroy the nested magnetic flux surfaces. Historically, the understanding of these mechanisms (and the direct comparison to theory and computational models) has been limited by diagnostic access. At the same time, it is known that in fluid dynamics, 2-D visualization of structures has greatly enhanced the level of understanding of the underlying physical mechanisms. In plasma physics, recent years have seen a tremendous advance in the application of imaging plasma diagnostics, which enable direct comparison to many theoretical models and detailed computer simulations for the first time. In this talk we discuss a number of these techniques, including the technical background of the diagnostic approaches and the underlying plasma phenomena being measured. Techniques to be discussed include Electron Cyclotron Emission Imaging (ECEI), in which focusing optics and frequency selection are used to form a 2-D image of local electron temperature, Gas Puff Imaging (GPI), in which line emission from neutral particles near the cold plasma edge are imaged with fast framing cameras, and 1-D arrays of magnetic probes, which enable direct visualization of wave dispersion relations.
- September 22
- Howard Baer, University of Oklahoma
- Host: Senarath de Alwis
- Title: Journey to the Dark Side (of the Universe)
- Abstract: An abundance of evidence shows that Standard Model (SM) particles
make up only one-fifth of the matter density of the universe,
while the remainder is likely some unknown elementary particle.
Supersymmetric theories of particle physics fix a host of problems
in the SM, while receiving some indirect support from experiment.
They also allow for at least three candidate particles for
cold dark matter (CDM) in the universe.
One possibility – the lightest neutralino (a WIMP) –
can be searched for 1. directly
via nuclear scattering of Big Bang relics in underground experiments,
2. via indirect searches for gamma rays or antimatter which results
from dark matter annihilations in the galactic halo, or 3. via direct
dark matter particle production at accelerators such as the
CERN LHC, which recently began collecting data.
Other dark matter possibilities include gravitinos (disfavored by Big
Nucleosynthesis) or a mixture of axion and axino particles.
While neutralinos are most popular, I show why mixed
axion/axino cold dark matter is most favored theoretically.
In this latter case, we would expect a discovery of supersymmetry at
along with a discovery of axions at the Axion Dark Matter Search
- September 29
- Immanuel Bloch, Max Planck Institute of Quantum Optics
- Host: Victor Gurarie
- Title: Exploring Strongly Correlated Quantum Matter in Artificial Crystals of Light
- Abstract: The realization of ultracold quantum gases at Nanokelvin temperatures has marked a milestone in modern quantum physics. With the help of laser light, these ultracold atom clouds can be stored in artificial periodic potentials created by laser light - so called optical lattices - that allow us to explore fundamental aspects of strongly interacting fermionic and bosonic quantum matter. In very recent experiments, we have been able to record single snapshots of a quantum fluid in which individual atoms are detected with single lattice site resolution. This open unprecedented novel opportunities for analyzing and manipulating strongly interacting quantum system. In my talk, I will review some of the recent experiments on strongly correlated quantum gases in optical lattices and highlight connections to condensed matter physics, quantum information science and atomic- and molecular physics.
Measured atom distribution on a lattice for a BEC (left) and Mott insulators (middle & right). Each point
marks the detected position of a single atom.
- October 6
- B.C. Low, National Center for Atmospheric Research
- Host: Dmitri Uzdensky
- Title: Magnetic topology and singularities in an electrically perfect fluid-conductor
- Abstract: During the motion of an electrically perfect fluid-conductor, electric current flows such that the flux of the embedded magnetic field is conserved across every fluid surface. By this property the magnetic flux surfaces and lines of force in a magnetohydrodynamic flow are just continuously deforming fluid surfaces and lines. This colloquium is aimed at an intuitive understanding of the nonlinear physics of such a flow in terms of the invariant topology of these surfaces and lines, leading to the Parker theory of spontaneous current-sheet formation in highly conducting fluids. In a real fluid, conductivity is high but not infinite. Thus the spontaneously forming current sheets must dissipate resistively as they tend towards the singularities implied in the ideal fluid picture. That a highly conducting plasma can at the same time heat itself resistively provides an attractive explanation of the million-degree hot solar corona.
- October 13
- Susan J. Seestrom, Los Alamos National Laboratory
- Host: Jerry Peterson
- Title: Science Challenges of Stockpile Stewardship
- Abstract: The U.S. Nuclear deterrent was designed in an era of underground nuclear testing. Since 1992 the U.S. has not engaged in nuclear testing and we rely on computer simulation and physics based models to ensure the safety and reliability of the stockpile. I will give an overview of the main science issues that drive our ability to carry out this work, and talk about the experimental tools we are using at Los Alamos to push forward the scientific underpinning of stockpile stewardship. I will also talk about our plans for evolving these capabilities going into the future.
- October 20
- Uwe Greife, Colorado School of Mines
- Host: Jerry Peterson
- Title: Applied Nuclear Physics in the Nuclear Science and Engineering Center of the Colorado School of Mines
- Abstract: This talk will present a short overview on the Applied Nuclear Physics program at the Colorado School of Mines. More in-depth, results from recent nuclear astrophysics related experiments using the DRAGON facility at Canada's National Laboratory for Nuclear and Particle Physics, TRIUMF, will be shown. Additionally, a small applied project using radioactive tracers in wear studies will be described.
- October 27
- Charles Kane, University of Pennsylvania
- Host: Leo Radzihovsky
- Title: Topological Insulators and Topological Band Theory
- Abstract: A topological insulator is a material that is an insulator on its interior,
but has special conducting states on its surface. These surface
states are unlike any other known two dimensional conductor.
They are characterized by a unique Dirac type dispersion relation and
are protected by a topological property of materials' underlying
bulk electronic band structure. These materials have attracted
considerable interest as a fundamentally new electronic phase with
applications from quantum transport to quantum computing. In this
talk we will outline the theoretical discovery of this phase and describe
recent experiments in which its signatures have been observed in both two
and three dimensional systems. We will close by arguing that the
proximity effect between an ordinary superconductor and a 3D topological
insulator leads to a novel two dimensional interface state which may
provide a new venue for realizing proposals for topological quantum
- November 3
- Murray Holland, University of Colorado
- Title: A sharper laser
- Abstract: I will present a new approach to lasers that promises to produce light
of unprecedented spectral purity and coherence, some two orders of
magnitude better than any system available today. The new idea is
based on superradiant emission, where an ensemble of atoms with an
extremely narrow atomic transition forms a macroscopic dipole, and
radiates collectively. This is quite unlike a typical laser where
atoms act independently. The resulting light source is expected to
have a spectral linewidth of just 1mHz and could lead to more accurate
and stable atomic clocks. Atomic clocks based on optical reference
transitions have improved tremendously in recent years, giving clocks
that tick 10^15 times per second, and can have a fractional stability
exceeding 10^16. That's a few seconds in age of our universe. But this
new sharper light source is essential to push the frontier even further,
so that fundamental tests of physics, such as the time variation of
constants and tests of gravity, might be possible.
- November 10
- Sean Shaheen, University of Denver
- Host: Ivan Smalyukh
- Title: Disorder, Defects, and Band Diagrams: Mechanisms of Operation of Bulk Heterojunction Organic Photovoltaics
- Abstract: Although the first reports of organic photovoltaic (OPV) devices were made over 50 years ago, it has been only recently that the power conversion efficiencies have become truly high enough to enable commercial application. At present OPV is a dynamic and fast moving field. The efficiency record, which now stands at 8.3%, is being set every few months by start-up companies in a heated competition. But underlying the excitement and potential of the technology is a rich body of nanoscale physics, chemistry, materials science, and electrical engineering that is far from fully understood.
In this talk I will focus on the physics of devices based on the bulk heterojunction approach, which relies on a nanoscale blend of electron-donating and electron-accepting molecules. These solution-processed devices can be made relatively inexpensively and rapidly. But there is a price to be paid: the active layer materials are intrinsically disordered in their morphology and heterogeneous in their optical and electronic structure. Understanding the correspondence between the disordered molecular structure and the resulting mesoscopic device properties such as charge carrier density, electric field distribution, etc. will be essential if device efficiencies are to approach thermodynamic limits. I will discuss a variety of experiments and simulations being undertaken by my group and collaborators aimed at understanding the morphological and energetic landscape of the blend. These include time-resolved photoluminescence and carrier transport measurements that reveal the disordered nature of the materials and also impedance spectroscopy measurements that show the presence of intrinsic, defect-induced carriers that lead to Schottky behavior at the electrodes. I will also discuss the origin of the open-circuit-voltage of the device and how it is impacted by ground state interactions. Lastly, I will discuss some more "exotic," multiphoton mechanisms that are capable in principle of yielding higher device efficiencies.
- November 17
- Bjoern Schenke, McGill University
- Host: Jamie Nagle
- Title: Understanding the hottest stuff on earth
- Abstract: In heavy-ion collisions at the Relativistic Heavy-Ion Collider at Brookhaven and, starting this month, at the Large Hadron Collider at CERN, extreme phases of nuclear matter are being produced. Under these conditions, theory predicts the creation of a new state of matter, the quark-gluon-plasma (QGP), which filled the entire universe only microseconds after the big bang.
I will review recent theoretical progress in describing the created matter, focusing on the development of a comprehensive simulation that is able to connect fundamental theory with experimental observations.
In particular, I will present two major building blocks of such an extensive simulation: the first 3+1 dimensional relativistic viscous hydrodynamic simulation of the bulk medium, and a Monte-Carlo simulation for the high-momentum probes, based on perturbative quantum-chromo-dynamics (pQCD) in a thermal medium. Combining the simulations, we can perform detailed comparisons with a wide range of experimental data, allowing for a quantitative characterization of the QGP and its interactions.
These studies will advance our understanding of strongly interacting many-body systems and the evolution of the early universe.
Colloquium schedules from previous semesters can be found here
Contact: Michael Hermele