Spring 2009 Physics Colloquium Schedule

University of Colorado Department of Physics

Colloquia are Wednesdays at 4 PM, Duane G1B20 unless otherwise noted

Coffee, tea, and cookies before all colloquia at 3:45 PM in Duane G1B31

Upcoming Colloquia:

January 14

Minhyea Lee, NIST Boulder
Host: Kyle McElroy
Title: Large Thermopower and Strong Correlation in Sodium Cobalt Oxides
Abstract: Electronic properties in strongly correlated systems often reveal strikingly enhanced features compared with those in conventional materials. Examples are the high transition temperature in cuprate superconductors and the colossal magnetoresistance in manganites. The layered triangular lattice cobaltates Na_xCoO₂ exhibit several interesting electronic phases such as superconductivity, charge ordering and high thermopower, as the Na content is varied. In this talk I will describe our experimental progress in the high Na doping range characterized by even steeper enhancement of thermopower, which makes this material a strong candidate for low temperature thermoelectric applications. Time permitting, I will also present recent data showing dramatic changes in transport upon very small variations in Na content, and discuss these findings in relation to a recent discovery of periodic Na ordering.

January 21

John Cary, University of Colorado
Title: Scientific Discovery by High-Performance Computation: Applications to Plasma Physics and Photonics
Abstract: Computation is now widely accepted as the third methodology for scientific inquiry, along with analytic theory and experiment. This talk will discuss some of the basics of computational scientific inquiry - what does it rely on? What skills are needed? How does it differ from the approaches of experiment and analytic theory? How can one have confidence, given the prevalence of bugs in software, that one is presenting valid results? Several examples of how computation has become critical in scientific investigation will be presented: (1) the discovery and analysis of the "bubble regime" of Laser Wake Field Acceleration, (2) inferring errors in "best experimental parameters" using computation, and (3) the discovery of new, photonic based accelerator cavities that can work with higher beam currents. A personal perspective on the next directions of computational physics will conclude the talk.

January 28

Please see posted flyers.

February 4

Oana Jurchescu, NIST Gaithersburg
Host: Kyle McElroy
Title: Building solid scientific foundations for future plastic electronics
Abstract: Organic electronic materials have raised interest in the academic and industrial communities owing to their wide range of properties. This class of materials promises to deliver new versatile electronic devices with various functionalities by combining diversity, flexibility, and light weight with the ease of processing and low cost. The field of organic electronics, also referred to as plastic electronics, has experienced dramatic progress in recent years. Its success has a strong impact on disciplines such as Physics, Chemistry, and Materials Science. Impressive steps were taken in understanding the fundamental issues that govern the physical processes, innovation in materials, processing methods, and device design. However, the performance, reproducibility, reliability, and stability of materials and devices remain a challenge, providing at the same time great research opportunities. I will discuss the current status of the field, including the accomplishments and the weaknesses in the plastic revolution. The key opportunities of plastic electronics that offer competitive advantages and interesting research resources will be highlighted. My contributions in the area of fundamental research needs of organic electronics will be emphasized. They include studies of structural and electronic properties of different single crystals and thin films, surface functionalization by nano-scale approaches, and investigation and control of various interfaces of interest in organic electronic devices (metal/organic, inorganic/organic, organic/organic).

February 9 Note special date: Monday at 4 PM, Duane G1B20

Kirill Bolotin, Columbia University
Host: Kyle McElroy
Title: Electrical transport and mechanical resonances in suspended graphene
Abstract: Graphene, a single atomic sheet of graphite, is distinguished from other electronic materials by its unusual band-structure rendering quasiparticles in it formally analogous to massless chiral fermions. However, many quantum limit transport phenomena in graphene remain yet to be observed due to the omnipresence of carrier scattering. We report a sample preparation method that yields high quality graphene specimens and demonstrates that much of the scattering in traditional graphene-on-silica devices is not intrinsic but rather results from the interaction with the substrate underlying the graphene. We fabricate devices where electrically contacted and electrostatically gated graphene samples are suspended over a substrate. Employing current-induced heating, we effectively remove the remaining impurities. The measured sample mobilities are found to exceed 200,000 cm²/Vs in such devices, an order of magnitude improvement over the best values reported in the literature. The very high mobility of our specimens allows us to observe previously inaccessible transport regimes in graphene.
Using the suspended graphene devices, we also demonstrate MHz frequency graphene resonators with all-electrical actuation and detection that are suitable for sensitive mass sensing. In these devices, the mechanical oscillations of suspended graphene are excited by a high-frequency voltage applied to a gate electrode, while the amplitude of the oscillations is monitored utilizing the transistor properties of graphene. The devices show resonances in the MHz range with high frequency tunability and robust signal levels. We further study the response of the devices to added mass, and confirm their potential as high-sensitivity mass detectors that can be regenerated by Joule heating.

February 11

Please see posted flyers.

February 16 Note special date: Monday at 4 PM, Duane G1B20

Mumtaz Qazilbash, University of California San Diego
Host: Kyle McElroy
Title: Mott transition in VO₂ observed by infrared spectroscopy and nano-imaging
Abstract: A grand challenge of contemporary condensed matter physics is the understanding of the emergence of metallic transport in correlated insulators or Mott insulators in which, for example, a temperature change or chemical doping induces anomalous conducting phases. Where charge, spin, orbital and phononic degrees of freedom result in competing interactions, exotic phases emerge including the pseudogap state in cuprates and manganites, high-temperature superconductivity, even phase separation in some manganites and cuprates. I will report on the electronic properties of a prototypical correlated insulator vanadium dioxide (VO₂) in which the metallic state is induced by increasing temperature. The pioneering technique of scanning near-field infrared microscopy allows us to directly image nano-scale metallic puddles that appear at the onset of the insulator-to-metal transition in VO₂. In combination with far-field infrared spectroscopy, the data reveal divergent optical mass in the metallic puddles which is evidence of a Mott transition. Our methods and results illuminate a new path towards spectroscopic exploration of electronic inhomogeneities in correlated electron systems.

February 17 Note special date: Tuesday at 4 PM, Duane G1B30

Tarje Nissen-Meyer, Princeton University
Host: Shijie Zhong
Title: Full-wave seismology from core to crust
Abstract: Seismologists study the physical phenomena of earthquakes and the propagation of seismic waves. In recent years, they have greatly benefitted from significant progress in digital data recording and collection, numerical techniques to account for the actual physics of wave propagation, and high-performance computing. Only now, this allows us to resolve scales of geophysical, environmental and exploration interests based on full-wave phenomena.
In this talk, I will introduce the principal constituents of seismological investigations, namely the physics of wave propagation, common approaches to data analysis, and various scales of interest. Specifically, I will explore such dualities as ray theory versus full-wave propagation, forward and inverse problems, at both planetary and local scales. Drawing upon above-mentioned progress and my related contributions, I will argue for approaching structural imaging from a purely full-wave point of view to potentially harness all available data with sophisticated techniques in a multiple-frequency, multi-scale fashion. By focusing on some of the most fascinating regions within the Earth such as lowermost mantle and shallow crust, I will explain how we render plausible 3D images for these areas with respectively optimized methods, depending on data availability and medium complexity. Finally, I will sketch future directions and likely ramifications for our understanding of the Earth.

February 18

Einat Lev, MIT
Host: Shijie Zhong
Title: Mantle anisotropy and the unearthing of Earth's internal flows
Abstract: The primary objective of the field of geodynamics is to explain Earth's internal deformation processes, the driving forces behind plate tectonics, earthquakes and volcanoes. Because it is impossible to make direct observations of the deformation taking place in the Earth's interior, geophysicists must rely on indirect observations made at the surface, such as the measurement of seismic anisotropy. When rocks deform, they can record the applied deformation by developing preferred orientation, which in turn leads to anisotropy.
The abundance of observations of anisotropy in the mantle raises the question - how is the mechanical anisotropy affecting the deformation? I present new results demonstrating the dramatic effect of accounting for anisotropic viscosity in various mantle flow scenarios, including subduction and lithospheric instabilities. My experiments show that including anisotropic viscosity is critical in many situations. This conclusion has profound implications for modeling of mantle deformation, as well as for the study of the deformation of other natural and synthetic anisotropic materials.

February 24 Note special date: Tuesday at 4 PM, Duane G1B20

Scott Hsu, Los Alamos National Laboratory
Host: Tobin Munsat
Title: Magnetized High Energy Density Laboratory Plasmas: Science & Applications
Abstract: High energy density (HED) plasma is a regime of matter that exists when the pressure applied to a material exceeds the energy density of the material at room temperature, corresponding to 1 Mbar (10^6 atm) or equivalently 10^11 J/m^3. A new generation of high power "drivers" (e.g., short pulse lasers or pulsed power systems) are now being utilized to create and study HED plasmas on university-scale facilities. As advocated by recent National Academy of Sciences and Department of Energy sponsored reports, new research opportunities now exist in the field of "High Energy Density Laboratory Plasmas" (HEDLP), which is needed for the development of inertial fusion energy (IFE), and which also has the potential to help address fundamental issues in astrophysics, nuclear physics, materials science, condensed matter physics, atomic/molecular physics, and beam physics. This colloquium will provide a brief overview of HEDLP, then focus on the science of magnetized high energy density laboratory plasmas (MHEDLP) and its main application, magneto-inertial fusion (MIF). MIF is a potential path to IFE in which a magnetic field in the fusion fuel is utilized to reduce thermal losses such that fuel compression requirements for fusion ignition are substantially relaxed. The talk will conclude with a description of a proposed experiment to generate HED plasmas via spherically convergent dense plasma jets formed by state-of-the-art plasma guns. If successful, this technique could provide a unique new way to repetitively form and study centimeter and microsecond scale HED plasmas, while enabling an advanced approach to MIF known as plasma liner driven MIF.

February 25

Bruno Zappone, University of Calabria
Host: Kyle McElroy
Title: Proteins on surfaces: adhesion, lubrication and self-assembly
Abstract: Human proteins accomplish highly specialized tasks, such as molecular recognition or catalytic reactions, that require specific protein composition and structure. However in situations often encountered in the "unfriendly" extra-cellular environment, proteins rely upon nonspecific molecular mechanisms related to their intrinsic polymeric nature--and undergo equally generic malfunctioning. I will present three examples from my recent research work where the study of non-specific surface properties of proteins may guide the development of novel biomimetic adhesives, anti-adhesive agents and lubricants for water-based applications.
(1) Contact lubrication of biological tissues involves glycoproteins that contain a long, unstructured central domain coated with short and highly hydrophilic sugar groups. I have considered lubricin, a glycoprotein of the articular joints. When adsorbed on solid surfaces in the form of "brush-like" layers, lubricin removes adhesion and can drastically reduce the friction coefficient to μ ∼ 0.03. The nanoscale mechanism of contact lubrication is analogous to what recently observed for synthetic polyelectrolytes and attributed to a lubricating fluid "sheath" of water molecules bound to the central hydrophilic domain.
(2) The resistance to fracture of the extracellular collagen matrix of bone is related to the presence of non-collagenous "glue" phosphoproteins such as osteopontin, that cross-link and bind to substrates and collagen bundles via Ca²⁺ -mediated salt bonds. Under a tensile stress, osteopontin is able to dissipate large amounts of energy by sequentially breaking Ca²⁺ bonds and elongating previously hidden flexible portions of the protein. Upon compression, the mechanism is partially reversed: mechanical energy is used to create and store bonds inside adsorbed protein layers.
(3) A common pathology of "mad cow," Alzeihmer's and Parkinson's diseases is the accumulation of anomalous protein filaments in tissues (amyloidosis), which is attributed to the aggregation of beta sheets the second most common secondary structure of proteins. Nanoscale images of the deposit left on different solid substrates by solutions of myoglobin and beta-lactogobulin (protein not related to any amyloid disease) reveal a spectacular proliferation of filaments that results from a non specific surface-induced misfolding of the protein.

February 26 Note special date and place: Thursday at 4 PM, JILA Auditorium

Christopher Holland, UC San Diego
Host: Tobin Munsat
Title: Testing our Understanding of Turbulence in Fusion Plasmas
Abstract: Addressing the challenge of global warming requires developing and deploying new sources of energy and electricty production with little or no C02 emissions. Nuclear fusion holds the potential to play a significant role as a long-term solution to this challenge. But in order to realize this potential, we need to understand the fundamental physics and dynamics of magnetized plasmas on many different temporal and spatial scales, so that we can interpret the results of current day experiments and optimize the design and configurations of future ones. In this talk, I will present a brief overview of the magnetic confinement approach to harnessing fusion energy, and the need for developing validated predictive models of plasma dynamics on many scales. I will also discuss current efforts in this area on the validation of gyrokinetic simulations of plasma turbulence, showing how recent advances in experimental diagnostics, computing capabilities, and computational models are allowing us to test first-principles based simulations of the nonlinear plasma dynamics at an unprecedented level. Future challenges and directions for these integrated studies of plasma dynamics will also be discussed, as well as the connection to non-fusion plasmas.

March 2 Note special date: Monday at 4 PM, Duane G1B20

Christian Degen, Leiden University
Host: Kyle McElroy
Title: Force detection of nuclear spins at the nanometer scale
Abstract: Magnetic resonance force microscopy (MRFM) engages ultrasensitive force detection to probe and image spins on nanometer lengthscales. Recently, this approach has led to the detection of a single electron spin [1] and to the demonstration of nuclear magnetic resonance imaging with a spatial resolution of 90 nm [2]. If we are to improve this resolution in order to eventually image on the scale of single nuclear spins, the sensitivity of the measurement must be improved by roughly 3 orders of magnitude.
In this talk I will discuss some work done in pursuit of this goal, including the design of nanoelectronic and nanomechanical devices to improve spin detection sensitivity and the significance of statistical spin polarization at nanometer lengthscales. I will then talk about recent efforts at imaging nanoscale biological objects, including three-dimensional MRI of individual tobacco mosaic virus particles with a spatial resolution of better than 10 nm. Finally I will review the recent proposal of diamond magnetometry as an alternative path to nanometer-scale MRI under ambient conditions.
[1] D. Rugar, R. Budakian, H. J. Mamin, and B. Chui. Nature 430, 329 (2004).
[2] H. J. Mamin, M. Poggio, C. L. Degen, and D. Rugar. Nature Nano 2, 301 (2007).

March 4

Hong Qin, PPPL
Host: Tobin Munsat
Title: How to compute plasma dynamics -- Geometric theory and algorithms
Abstract: Plasma physics is the physics foundation for modern and next-generation fusion devices, such as ITER, while the theoretical methods of plasma physics remain in the domain of classical physics. Due to its multiscale nature, the nonlinear dynamics of plasma in a magnetic field is complex and only accessible through large-scale computer simulations. Standard, off-the-shelf algorithms for dynamic systems in general do not have the efficiency and long-term accuracy needed for plasma simulations. It turns out that optimized algorithms for plasma dynamics can be designed by reformulating classical plasma physics theory using the modern language of differential geometry and field theory. Indeed, an elegant theory is a good algorithm at the same time.

March 9 Note special date: Monday at 4 PM, Duane G1B20

Merav Opher, George Mason University
Host: Tobin Munsat
Title: Plasma Effects in the Interaction of the Solar System with the Interstellar Medium
Abstract: Plasma effects are ubiquitous and known to be crucial in space and astrophysical media; these media are excellent plasma laboratories and provide observational data that add valuable constraints to theoretical models. Modern computational techniques such as magnetohydrodynamic MHD and kinetic modeling are currently used to explore several fundamental plasma effects such as turbulence, reconnection, shocks evolution, etc. In this talk I will discuss one example of how cross-fertilization between basic plasma physics, sophisticated computational work allied with detailed observational data can be guided towards resolving important physical phenomena. The twin Voyager spacecraft, both approximately 100 AU from the Sun, are providing us with an unexpected view of how stars interact with their surrounding media. For the first time we are able to make in situ measurements of particles and fields at the boundaries of the solar system. Although our understanding of collisionless shocks has been advanced in recent years, we still lack a comprehensive study on how the evolution of shocks (and their characteristics) is affected by the environment in which they propagate. From the in situ observations of the termination shock by Voyager 1 and 2, it became clear that shock properties are much more complex than previously expected. Voyager 1 crossed the termination shock in Dec 2004, and is now in the heliosheath, and in August 2007 Voyager 2 also crossed the shock. In this talk I will review our recent work where we were able to constrain the direction of the local interstellar magnetic field. As a result of the interstellar magnetic field, we have shown that the solar system is asymmetric, being pushed toward the Sun in the southern hemisphere. I will review as well our other work exploring reconnection effects, near the heliopause (the region that separates the solar system and the interstellar media) and waves, instabilities and turbulence in the heliosheath (the regions where the solar wind is subsonic). These effects will be able to be sampled by future Voyager observations, providing direct testing of the theories we have developed. I will discuss these results of our understanding of shocks, particle acceleration and implication for plasma phenomena in laboratory.

March 11

Dmitri Uzdensky, Princeton Department of Astrophysics
Host: Tobin Munsat
Title: Magnetic Reconnection, from Slow to Fast: Current Understanding, Applications, and Open Issues
Abstract: Magnetic reconnection is an important basic plasma-physics process associated with a rapid change of magnetic field topology. It often leads to a violent release of magnetically-stored energy and thus powers many of the most spectacular and energetic phenomena in laboratory, space, and astrophysical plasmas.
In this talk, I will review the present status of our knowledge of magnetic reconnection, with an emphasis on the transition from the slow collisional to the fast collisionless reconnection regime. I will then describe some of its important solar- and astrophysical applications and will show how the application of the recent advances in reconnection research can help us understand the heating of the solar and black-hole accretion-disk coronae. I will conclude by outlining some of the important open questions and the directions of future reconnection research.

March 12 Note special date and place: Thursday at 12 PM, JILA Auditorium

Lowell Miyagi, UC Berkeley
Host: Shijie Zhong
Title: Understanding Deformation in the Deep Earth through Texture and Anisotropy
Abstract: Texture or preferred orientation of anisotropic crystallites within a polycrystal can result in bulk anisotropy. This has an affect on a wide range of physical properties such as viscosity, heat flow, electrical conductivity, and elastic properties. Texture has many applications, for example ceramics for high temperature superconductors, polymers, sheet metals for automobiles and even beverage cans are texture engineered for optimum performance. Texture is of great interest to geophysics due to observations that large regions of the Earths interior exhibits anisotropic propagation of seismic waves. It is likely that this seismic anisotropy is due to texturing associated with plastic deformation in the Earths interior. As large scale geologic processes such as mantle convection and slab subduction are associated with plastic deformation, knowledge of the deformation behavior of deep Earth mineral phases is of critical importance to understand the dynamic behavior of the Earths interior as well as assess observed seismic anisotropy. My research attempts to link mineral physics experiments to geodynamics and seismic observations. I combine laboratory deformation experiments to measure and quantify texture development with polycrystal plasticity modeling. Samples are deformed with high pressure and temperature deformation devices and textures are measured in-situ at national synchrotron x-ray facilities using high brilliance x-rays and radial diffraction. This yields information on texture development for samples deformed under well controlled deformation conditions and geometries. Polycrystal plasticity modeling of these results elucidates active deformation mechanisms. Once these mechanisms are understood this information can be combined with geodynamic modeling to predict the development of texture and seismic anisotropy in the deep Earth. I will discuss applications of these techniques to the major lower mantle mineral phases of (Mg,Fe)SiO3 perovskite, post-perovskite, and (Mg,Fe)O ferropericlase.

March 12 Note special date and place: Thursday at 4 PM, JILA Auditorium

Dmitry Reznik, Forschungszentrum Karlsruhe
Host: Dan Dessau
Title: Phonons as a Probe of Exotic Physics in Correlated Electron Systems
Abstract: Investigation of crystalline solids with complex electron-electron and electron-lattice interactions is one of the most promising routes towards novel states of matter and functional materials. Many of these are now basically understood (e.g. the Fermi liquids), whereas others constitute unsolved fundamental problems such as heavy fermions, high temperature superconductivity, colossal magnetoresistance, and exotic phases in itinerant magnets. Recent inelastic neutron and x-ray scattering measurements of phonons revealed exceptionally strong electron-phonon coupling in many of these materials, which is often unexpected from conventional theory. I will show how such experiments serve as a novel probe of exotic physics in correlated electron systems using a high temperature superconductor, La_2-xSr_xCuO₄, as an example. This technique is particularly promising due to ongoing huge investments in new neutron and x-ray sources and instruments worldwide.

March 16 Note special date: Monday at 4 PM, Duane G1B20

Kramer Akli, General Atomics, San Diego CA
Host: Tobin Munsat
Title: From High Energy Density Physics To Fast Ignition
Abstract: High Energy Density Physics (HEDP) is an exciting and rapidly growing field of research. High energy density states of matter are found throughout the universe (supernovae, stellar interiors, cores of large planets). In the laboratory, these states of matter can be created with high power lasers, particle beams, and Z-pinch generators. This field of research has been mainly driven by nuclear fusion, which promises a reliable, clean, and safe energy source. In this talk I will introduce conventional inertial confinement fusion (ICF), also known as hot spot ignition. I will discuss the fast ignition scheme, an alternate concept in inertial confinement fusion. Fast ignition uses two separate lasers: a high-energy long-pulse (few ns) to compress a spherical capsule of deuterium-tritium and a high-intensity short-pulse (10-20 ps) laser to ignite it. This approach has the advantages of higher energy gain and relaxed symmetry requirements over conventional hot spot ignition. I will show examples of basic science conducted with university-scale high intensity short pulse lasers in the fast ignition context.

March 18 Joint APS and Physics Department Colloquium

Jeff Brack, Colorado State University
Host: David Bartlett
Title: The Pierre Auger Observatory: Ultra High Energy Cosmic Rays and Experimental Physics at 10²⁰ eV
Abstract: The highest energy particles known are cosmic rays that have been observed hitting the earth's upper atmosphere with energies above 10²⁰ eV. The acceleration mechanism for particles at these energies is uncertain. It is likely that these particles are protons, which in space lose energy rapidly through interactions with the 2.7K microwave background radiation, resulting in pion production. The implication is that sources of very high energy particles exist at cosmologically small distances from earth.
The Pierre Auger Observatory's southern array is now completed, covering 3000 km² in western Argentina. Integrated exposure over the last 4 years, during construction and after completion last year, is nearly double that of previous experiments combined. Early results indicate that UHECR arrival directions are correlated with nearby extra-galactic sources. The southern observatory will be described, along with recent results and future plans for the larger Auger North Observatory in Colorado.

March 25: No colloquium: Spring Break

April 1

Tim Donaghy, Union of Concerned Scientists
Host: Scott Robertson
Title: Protecting the Integrity of Federal Government Science
Abstract: The federal government runs on vast amounts of information, and makes policy decisions every day that affect the health and well-being of all Americans. Although science is rarely the only factor driving public policy, scientific input should always be weighed from an impartial perspective. Unfortunately, numerous independent investigations have documented a pattern of suppression, manipulation, and distortion of federal science before it enters the policy process. Under the George W. Bush administration, political interference in science became pervasive and systemic. Furthermore, recent changes in the structure of the federal government impair the ability of federal scientists to fulfill their responsibility to serve their agencies and the public interest. The talk will focus on the steps that should be undertaken by President Obama and the 111th Congress to halt these abuses and to implement reforms and safeguards to prevent them from recurring.

April 2 Note special date and place: Thursday at 4 PM, Humanities 250

David Kaiser, MIT
Host: Allan Franklin
Title: How the hippies saved physics
Abstract: In recent years, the field of quantum information science--an amalgam of topics ranging from quantum encryption, to quantum computing, quantum teleportation, and more--has catapulted to the cutting edge of physics, sporting a multi-billion-dollar research program, tens of thousands of published research articles, and a variety of device prototypes. This tremendous excitement marks the tail end of a long-simmering Cinderella story. Long before the big budgets and dedicated teams, the field moldered on the scientific sidelines. In fact, the field's recent breakthroughs ultimately owe their origins to the hazy, bong-filled excesses of the 1970s New Age movement. Many of the ideas that now occupy the core of quantum information science once found their home amid an anything-goes counterculture frenzy, a mishmash of spoon-bending psychics, Eastern mysticism, LSD trips, CIA spooks chasing mind-reading dreams, and comparable "Age of Aquarius" enthusiasms. For the better part of two decades, the concepts that would, in time, blossom into developments like quantum encryption were bandied about in late-night bull sessions and hawked by proponents of a burgeoning self-help movement--more snake oil than stock option. This talk describes the field's bumpy transition from New Age to cutting edge.

April 6 Note special date and place: Monday at 4 PM, Duane G1B31

Sergio Speziale, GFZ, Potsdam, Germany
Host: Shijie Zhong
Title: Elastic properties of (Mg,Fe)O ferropericlase in Earths lower mantle
Abstract: The objective of mineral physics understanding the physical properties of the materials that constitute the Earth interior is fundamental to construct a quantitative model of our planet, which is a prerequisite for policies of rational utilization of Earth resources and monitoring sources of natural hazards. Geophysical methods, especially seismology, give us a large scale picture of the deep Earth, which has to be interpreted based on the properties of the candidate minerals in order to produce a physical model that is consistent with compositional constraints. In the last few years, advancements in global seismology and mineral physics have substantially affected our view of the lower mantle, the region between 660 and 2900 km depth. The existing paradigm about lower mantle mineralogy has been modified by the discovery of a polymorphic phase transition of its most abundant silicate component. Very recently, the observation of pressure-induced electronic spin pairing of Fe in the major phases of the lower mantle has raised new questions about the real effect of Fe-Mg substitution in these minerals. In fact, existing views about their elastic properties are mainly based on extrapolations of low-pressure data for isostructural iron-free compounds. The physical properties of ferropericlase (Mg,Fe)O, the second most abundant mineral of Earths lower mantle, are strongly affected by the pressure-induced modification of the electronic structure of Fe2+. I will present the results of my research on the density and elastic properties of ferropericlase at pressures of the lower mantle and their implications for interpreting controversial aspects of seismologic models of wide regions of the deep Earth.

April 8

Jose P. Mestre, University of Illinois at Urbana-Champaign
Host: Noah Finkelstein
Title: From Expert-Novice Reasoning to Classroom Practice: Pure and Applied Studies of Physics Cognition
Abstract: I will discuss two studies in physics cognition at the opposite ends of the pure-applied spectrum. One study explores a phenomenon known as change-blindness in visual cognition, which focuses on whether or not unsuspecting participants notice changes made in real-time to diagrams or visual scenes, and the factors contributing to noticing. In our experiment, physics experts and novices viewed a physics diagram on a screen and were asked to explain the physics underlying the situation; however, the diagram was changed during explanation while participants were slightly distracted to ascertain whether or not they noticed the change in the diagram. I will discuss the types of changes that are noticed (and not noticed) by experts and novices, as well as provide the background for why a study such as this sheds like on the cognitive functioning of physics experts and novices. The second, more applied study explores the efficacy of three different methods for delivering physics content (electricity) as preparation for attending interactive lectures that focus on concept refinement as opposed to dispensing information: 1) web-based multimedia learning modules, 2) scripts of those modules, and 3) standard textbooks. We found significant differences in amount of learning and retention among these three types of delivery systems. I will discuss the design of the materials, the underlying cognitive theory, the experiment, and findings.

April 15

Phil Bucksbaum, Stanford
Host: Henry Kapteyn
Title: Femtosecond x-rays and gigavolt fields in AMO Physics
Abstract: Within the next few days, scientists at the SLAC National Accelerator Laboratory at Stanford are expected to inaugurate the world's first x-ray free electron laser. Ultrafast table-top lasers are already routinely producing sub-femtosecond vuv harmonic radiation. These new femtosecond sources are transforming research in atomic and molecular physics. They are tools to visualize and control the electrons that determine the structure and properties of atoms and molecules.

April 16 Note special date, time and place: Thursday at 1:30 PM, JILA Auditorium

Stojan Radic, University of California San Diego
Host: Dana Anderson
Title: Overcoming Parametric Stochastic Barrier
Abstract: Parametric interaction in high-confinement fiber was recently used to set new records in frequency conversion, band-invariant amplification and fast coherent signal processing. In contrast to conventional four-photon mixing platforms, new classes of fibers are capable of net gain mixing over ranges exceeding 100THz, spanning visible to infrared bands. While ultrawideband wavelength conversion represents the most widely recognized application, immediate future holds exciting prospects in ultrafast, real-time processing and coherent control.
Current generation of high-confinement fiber is fabricated with exceeding precision: its transverse variations are expressed in multiples of silica molecular diameters. With nanometer-scale radial precision maintained over kilometers, high-confinement fibers stand among the most precisely fabricated structures in modern engineering. Unfortunately, even molecular-scale core fluctuations pose a basic barrier: an arbitrary-wide mixer cannot be constructed from randomly fluctuating waveguide. In simple terms, small uncertainties in waveguide geometry have destructive impact on long-scale phase matching between interacting optical waves. This fundamental limitation is known as stochastic parametric barrier and is principal obstacle on a path to practical parametric device construction.
Rather than insisting on unphysical waveguides (requiring sub-molecular radial control), an alternative approach is required. Indeed, it is possible to map nanoscale fiber fluctuations exactly and then use the information to synthesize arbitrary mixer response. To accomplish this, we introduced new energy delivery method based on localized four-photon mixing. The technique improves the sensitivity of existing dispersion mapping methods by orders of magnitude and is applicable to arbitrary waveguide type. The talk will describe the effort that led to the ability to sense molecular-scale geometry variations along km-long fiber for the first time. Implications of the new technique will be illustrated on general mixer applications.

April 21 Note special date, time and place: Tuesday at 1 PM, JILA Auditorium

Alberto Marino, NIST Gaithersburg
Host: Dana Anderson
Title: Entangled Images: Generation and Delay with Four-Wave Mixing
Abstract: I will present experimental studies that we have carried out on the generation and delay of highly entangled beams of light, know as twin beams. The quantum correlations present in twin beams have recently generated great interest due to their applications in quantum information, quantum imaging, and quantum computing. In the first part of the talk I will show that non-degenerate four-wave mixing (4WM) in a rubidium vapor cell is an excellent source of continuous-variable (CV) entangled twin beams, with an intensity-difference noise of less than 13% of the classical shot-noise level. Unlike other systems that rely on the use of a cavity, the system that we use can support a large number of spatial modes. This leads to spatial quantum correlations and makes it possible to produce CV entangled images. In the second part I will show that, in addition to generating entangled twin beams, the 4WM process in a vapor cell can act as a tunable delay line for CV entanglement without significant degradation. This has allowed us to delay the entangled images.

April 22

Cristina Marchetti, Syracuse University
Host: Meredith Betterton
Title: Swimming with a crowd: the collective behavior of soft active matter
Abstract: Bacterial suspensions and extracts of cytoskeletal filaments and motor proteins are examples of assemblies of interacting self-driven units that form a new type of active soft matter with intriguing collective behavior. Active systems self-organize in fascinating patterns and exhibit novel long range correlations, can generate forces and adapt their structure and rheological properties in response to stimuli. In this talk I will review our work on using nonequilibrium statistical physics to derive a continuum description of active matter from specific models of single particle dynamics.

April 23 Note special date, time and place: Thursday at 1 PM, JILA Auditorium

Hakan Tureci, Institute for Quantum Electronics, Zurich, Switzerland
Host: Dana Anderson
Title: Lasing in complex photonic media
Abstract: Micro- and nano-structured light-confining media for lasing applications are characterized by a certain degree of spatial complexity and openness. A few representative examples are photonic crystal defect cavities, mesoscopic dielectric resonators and disordered light-scattering media. These recently introduced coherent light sources have challenged our understanding of lasers and underlined the absence of a predictive theory of lasing in such complex media.
In my talk, I will describe an 'ab initio' self-consistent (AISC) approach to access the steady state properties (frequencies, thresholds, internal and external fields) of an arbitrarily complex laser structure from a few simple inputs of the resonator and the gain medium. This approach overcomes two of the essential obstacles faced by time-independent approaches: the correct description of an arbitrary degree of leakiness of the light-confining structure and the spatial hole-burning interactions in the multi-mode regime. It does so by introducing an appropriate set of modes, namely the set of "Constant Flux Modes", to correctly describe the steady state response of the linear scattering system and a self-consistent set of equations to treat the lasing non-linearity in the multi-mode regime to infinite order. The lasing properties of diffusive random lasers, a case that presented a formidable challenge to conventional laser theory was recently addressed using the AISC theory successfully (Science 320, 643, 2008). I will present a brief sketch of the basic framework of AISC and a few representative applications to design of power-efficient and functional micro and nano-lasers.

April 28 Note special date, time and place: Tuesday at 1 PM, JILA Auditorium

Marian Florescu, Princeton University
Host: Dana Anderson
Title: New classes of non-crystalline photonic band gap materials
Abstract: Due to their ability to control the most fundamental properties of light, photonic band gap materials open a new frontier in both basic science and technology. Until now, the only materials known to have complete photonic band gaps were photonic crystals, periodic structures. In this talk, I will show that there exists a more general class of systems, called hyperuniform photonic structures, which exhibit large and complete photonic band gaps. This classification includes not only crystalline structures, but also non-crystalline materials, ranging from isotropic, translationally-disordered structures to quasicrystals with crystallographically-forbidden rotational symmetries. I will show that both the periodic and aperiodic photonic-band gap structures can be systematically generated from hyperuniform point patterns through a general tessellation protocol. The newly discovered systems have distinctive structural and optical properties and offer advantages for many applications including isotropic light sources, lossless waveguides with arbitrary bending angles, or flexible optical insulator platforms for optical circuits.

April 29

Dirk Morr, University of Illinois at Chicago
Host: Kevin Stenson
Title: Everything Nano
Abstract: The nanoscale provides a fascinating testing ground not only for fundamental concepts in quantum mechanics, but also for our understanding of how complex physical phenomena emerge on the way from single atoms to macroscopic systems. In this talk, I will discuss some of the most intriguing and unique phenomena that are observed at the nanoscale, such as quantum imaging and quantum invisibility. In addition, the creation of nanoscopic replicas of strongly correlated electron systems has opened new venues for unraveling some of the most puzzling problems in condensed matter physics. In particular, I will show how these nanoreplicas can provide insight into the relation between quantum criticality, competing phases, and non-Fermi liquid behavior seen in heavy fermion materials and cuprates superconductors.

May 6 Joint APS and Physics Department Colloquium

Note special location: JILA Auditorium

William J. Weber, University of Trento, Italy
Host: Peter L. Bender
Title: The LISA Pathfinder Mission: preparing for gravitational wave observations in space
Abstract: The Laser Interferometer Space Antenna (LISA) will open the observational window of space-based gravitational wave astronomy at frequencies down to 0.1 mHz. Achieving the targeted high resolution observations of distant coalescing massive black holes and local Milky Way binary systems will require geodesic reference test masses that are in perfect free-fall to less than femto-g/sqrt(Hz) acceleration noise, and that serve as interferometry reference end mirrors with position measured to within tens of pm/sqrt(Hz). The LISA Pathfinder mission, scheduled for launch in 2011, aims to demonstrate such a geodesic motion reference at levels approaching the LISA goals, with a test mass inside a co-orbiting "drag-free" spacecraft and hardware -- including test mass, surrounding electrostatic sensor and force actuator, local interferometry position readout, and spacecraft micro-thrusters -- that can be used on LISA. I will discuss the LISA Pathfinder flight experiment and what we have learned, in ground-based preparations for the mission, about the limits on space-based gravitational wave detection and on other precise experiments that demand near-perfect free-fall.

Colloquium schedules from previous semesters can be found here

Contact:

Michael Hermele