Spring 2008 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 regular colloquia at 3:45 PM in Duane G1B31

Upcoming Colloquia:

Colloquia that have already occurred:

January 16

Robin Santra, Argonne National Lab
Host: Chris Greene
Title: X-ray physics at high intensity

Abstract: In textbook x-ray physics, all phenomena may be understood in terms of either one-photon absorption or one-photon scattering processes. There are two obvious ways we can bring higher-order processes into play. The first approach is to apply strong optical laser fields and effectively modify the way x rays interact with matter. This allows us to use x rays to gain deeper insight into strong-field processes. In addition, optical lasers may be utilized to control the propagation of x rays through a medium. Gas-phase systems are particularly suitable for illustrating the basic principles underlying combined x-ray and laser interactions. Topics addressed include orbital alignment of atomic ions produced in a strong laser field; electromagnetically induced transparency in the x-ray regime; and laser-induced alignment of molecules. The second approach is to introduce nonlinearities in the x-ray/matter interaction itself. Currently available x-ray sources are not intense enough for this purpose. But future x-ray free-electron lasers will allow us to explore the novel area of x-ray nonlinear physics. For instance, x-ray free-electron lasers are expected to be intense enough to drive inner-shell excitations so rapidly that core-hole formation takes place on the femtosecond time scale, i.e., on the time scale of Auger decay leading to core-hole relaxation. It is shown how x-ray photon statistics influences two-photon-induced double-core-hole formation.

January 22 Note special date and place: Tuesday at 4 PM, JILA Auditorium

Andreas Becker, Max Planck Institute
Host: Chris Greene
Title: Molecules in intense laser fields

Abstract: The development of ultrashort laser pulse technology has made possible the generation of intense light fields whose magnitudes exceed that of the Coulomb field within atoms and molecules. The advent of such pulses provide the opportunity to affect, image and control the motion of electrons in such quantum systems in space and time on a sub-femtosecond and sub-Angstrom scale. Current available laser intensities can be made so high to saturate the probabilities of ionization of an atom or molecule to high charge states. Unexpectedly, it has been observed that almost all molecules are harder to ionize, i.e. exhibit lower ionization probabilities, than the so-called companion atoms having equal ionization energies. A theoretical analysis of this phenomenon of suppressed molecular ionization along with numerical results for diatomics and fullerenes will be presented. In the second part of the talk it will be discussed how electron dynamics in a molecule can be controlled via the oscillating electric field of a few-cycle laser pulse. As an example, numerical simulations exhibiting electron localization in the dissociation of the hydrogen molecular ion will be shown.

January 23

Suliana Manley, National Institutes of Health
Host: Meredith Betterton
Title: Exploring protein organization and dynamics in model membranes and cells

Abstract: Cell membranes are characteristically heterogeneous, both structurally and dynamically. The spatial arrangement and dynamic movement of proteins in cell membranes are essential for coordinating many cellular functions, contributing to the specificity and sensitivity of a cell's response to its environment. I will discuss two routes toward understanding these heterogeneities, using either model membranes or mammalian cells. In the controlled environment of model membranes, we studied the influence of membrane-tethered proteins and lipid compositions on membrane organization. We found that protein and lipid organization in this simple model system were coupled. While heterogeneities in model membranes can be large enough to measure using diffraction-limited optical microscopy, heterogeneities in cell membranes generally exist on smaller scales. We addressed this by combining the super-resolution technique of photoactivated localization microscopy (PALM) with live cell single particle tracking (SPT). Photo-switchable, genetically expressed probes are used to create spatially resolved maps of single-molecule motions. We explored the capabilities of this method by imaging the membrane proteins Gag and VSVG, obtaining up to thousands of trajectories per cell, several orders of magnitude more than enabled by traditional SPT. This new method allows us to correlate structural and dynamic heterogeneities at the molecular scale.

January 28 Note special date: Monday at 4 PM, Duane G1B20

Erik Brubaker, University of Chicago
Host: Eric Zimmerman
Title: Weighing truth: A precision measurement of the top quark mass

Abstract: One of the great successes of the collider physics program at Fermilab has been the discovery and study of the top quark, whose large mass suggests it could play a central role in electroweak symmetry breaking and the origin of mass. Through radiative corrections the top quark mass enters into precision tests of the consistency of the standard model of particle physics, in particular constraining the mass of the putative Higgs boson. I will describe a precision measurement of the top quark mass using the CDF II detector at Fermilab's Tevatron, touching on some of the experimental challenges we face and the analysis techniques we employ. I will also show the first direct experimental upper limit on the top quark decay width.

January 29 Note special date and place: Tuesday at 4 PM, JILA Auditorium

Tommy Angelini, Harvard
Host: Meredith Betterton
Title: Scratching the Surface of Condensed Cellular-Matter Physics

Abstract: The collective motion in dense communities of individual organisms such as flocking birds, marching ants, and swarming fish, is not only captivating to the casual observer, but has long interested biologists, physicists, and even sociologists, as they explore how system-wide coherent behavior emerges from basic properties of the single units. This collective dynamics perspective is rarely applied to condensed groups of eukaryotic cells, despite the fact that each day we all benefit from cells working together, as our tissues and organs perform high level functions. Is tissue function an emergent phenomenon? In my talk, I will not attempt to answer this question, but I will present studies of simple systems that serve as models of collective cellular dynamical behavior, with the goal of building a statistical mechanics of condensed cellular-matter. I will show that short-lengthscale fluctuations, within confluent MDCK and Cos-7 epithelial cell layers, propagate like acoustic waves, and that the speed of these microscopic fluctuations play a role analogous to k_BT, scaling with macroscopic parameters, such as spatial correlation lengths and cell density.

January 30

Alysia Marino, University of Toronto
Host: Eric Zimmerman
Title: Beaming Neutrinos from Sea to Sea

Abstract: Over the past decade compelling evidence has emerged that neutrinos have non-zero masses and that neutrinos change from one flavour to another. Intense neutrino beams generated by particle accelerators are now being used in order to more precisely probe the spectrum of neutrino masses and mixing.
This talk will describe current and planned neutrino beams, especially the Tokai-to-Kamioka (T2K) experiment, now under construction in Japan. T2K will examine a beam of muon neutrinos produced at J-PARC on the east coast of Japan. With two neutrino detectors, one located near the origin of the beam, and another detector located 295 km away, T2K will look for the disappearance of muon neutrinos and the appearance of electron neutrinos over a long distance. T2K will begin taking data in 2009.

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

Elliot Lipeles, University of California at San Diego
Host: Eric Zimmerman
Title: An Introduction to Higgs Hunting

Abstract: The Higgs particle is the keystone of our understanding of the electromagnetic and weak forces, but it has not yet been observed. In this talk, I will describe the role it plays in the standard model of particle physics and describe experiments targeted at finding out if it really exists.

February 6

Michael Spanner, University of Toronto
Host: Chris Greene
Title: Coherent control of reactive scattering: From entanglement to attosecond collisions

Abstract: Coherent control seeks to use quantum interference effects to control the outcome of atomic and molecular processes. After a brief outline of the basic principles and methods of coherent control, the vast majority of which are applied to photo-initiated processes, I investigate one of the few existing proposals for the coherent control of reactive scattering, previously believed to require the presence of entanglement between the scattering partners. It is found that the role of entanglement in this scenario is to setup correlations between the scattering pair such that all collisions in the scattering volume occur for a single internal quantum superposition of the incident particles. This new insight then allows for the identification of a second regime of coherently controlled reactive scattering that relies on timing-based correlations, as opposed to entanglement. A proof-of-principle experiment of this latter timing-based scenario is proposed using strong field electron recollision in molecular hydrogen.

February 12 Note special date and place: Tuesday at 4 PM, Duane G1B30

Ajit Joglekar, University of North Carolina
Host: Meredith Betterton
Title: Elucidation of the 3-D protein architecture of a kinetochore-microtubule attachment

Abstract: Concerted movement of chromosomes is essential for their accurate segregation during cell division. The force necessary for this movement is generated by a complex protein machine assembled at a unique site on each chromosome, known as the kinetochore. The kinetochore links a chromosome to the plus-ends of one or more spindle microtubules, thus coupling chromosome movement to the growth and shortening at these ends. It also ensures coordination between chromosome movement and the cell cycle by participating in the spindle assembly checkpoint. This functional complexity is reflected in the protein structure of the kinetochore — it is built from more than 40 proteins assembled into distinct sub-complexes. The goal of my post-doctoral research is to understand the arrangement of proteins within an individual microtubule attachment site at the kinetochore in 3-D.
The protein architecture of a microtubule attachment site at the kinetochore can be revealed by answering two questions: (1) how many copies of each protein complexes exist in one kinetochore-microtubule attachment, and (2) what is the arrangement of protein complexes within the microtubule attachment site. The relatively simple kinetochore in budding yeast (Saccharomyces cerevisiae), which attaches to only one microtubule plus-end, provides an excellent model to address these questions. I have counted the copy number of each protein complex at the budding yeast kinetochore using an in vivo technique of quantitative fluorescence microscopy of GFP-tagged kinetochore proteins. To determine the relative localization of each complex within the kinetochore, I have adapted Single molecule High Resolution Co-localization (SHReC), an in vitro technique, for use in vivo. Using this technique, I am currently assembling a protein localization map for the yeast kinetochore with ~ 10 nm accuracy. Using the protein counts and localization data along with available biochemical and structural information for each protein complex, I will construct a 3-D visualization of the kinetochore-microtubule attachment. Finally, analysis of protein counts in two other fungal systems (fission yeast and Candida albicans) established that complex kinetochores with multiple microtubule attachments are built by repeating an evolutionarily conserved subunit structure that is equivalent to the budding yeast kinetochore.

Most of the core kinetochore proteins are conserved in all eukaryotes, from budding yeast to humans. Therefore study of the budding yeast kinetochore can provide the foundation for quantitative models describing the complex behavior and regulation of the human kinetochore. The 3-D protein architecture assembled in this study will provide the molecular context for future in vivo studies investigating the biophysical mechanisms underlying the basic functions of a kinetochore-microtubule attachment such as force generation, and potential factors (e.g. an opposing force) that regulate its activity.

February 13

Benjamin Kilminster, Ohio State University
Host: Eric Zimmerman
Title: Fermilab's race for the Higgs boson

Abstract: One of the most important mysteries in our understanding of the universe is how elementary particles acquire mass. Our best explanation for this requires the existence of a particle called the Higgs boson, which has not yet been directly observed. Particle physicists at Fermilab, near Chicago, are currently capable of producing and detecting Higgs bosons from collisions of matter and antimatter at very high energies. I will explain what exactly these physicists are looking for, and present the experimental challenges involved in a few particular methods for differentiating Higgs bosons from other background processes. Finally, I will discuss future prospects for Higgs boson discovery at Fermilab, as well as the discovery potential of future experiments.

February 18 Note special date: Monday at 4 PM, Duane G1B20

Florencia Canelli, Fermi National Accelerator Laboratory
Host: Eric Zimmerman
Title: Top Quarks and the High Energy Frontier

Abstract: One of the still missing pieces of the Standard Model of particle physics is the Higgs boson, providing a mechanism to generate the masses of the particles. This talk will explore aspects of experimentation at the High Energy frontier, starting from experience at the Tevatron accelerator currently providing the world's highest energy particle collisions. In particular, a precision measurement of the heaviest fundamental particle, the top quark, using the Collider Detector at Fermilab (CDF) will be presented. An outlook will be given towards a direct search for the Higgs boson at the Tevatron and beyond.

February 20

Alberto Bilenca, Harvard Medical School
Host: Meredith Betterton
Title: Optical Bio- and Nano- Imaging: New Concepts and Applications

Abstract: In the first part of my talk, an intriguing approach of fluorescence microscopy termed Fluorescence Coherence Tomography (FCT) will be introduced. Unlike classical fluorescence imaging techniques, FCT utilizes phase information of fluorescent waves (rather than intensity) to provide new avenues for nano-imaging, nano-sensing and comprehensive bio-imaging of large samples. I will show that FCT actually solves the fundamental, long-lasting trade-off between axial resolution and depth-of-field in conventional fluorescence microscopy by achieving micrometer-scale axial resolutions over submillimeter- to millimeter-size scales. These unique properties of FCT significantly improve ones ability to acquire manageable-sized cross-sectional images at the organism-level in a single shot. In addition, I will present extensions of FCT to imaging and sensing at the nanoscale.
In the second part of my talk, I will address an important phenomenon in Optical Coherence Imaging (OCI)—speckle. I will discuss the limits to the information content of OCI images of biological samples arising from speckle and scatter. The understanding of such limits is of fundamental interest, especially because the technological advances in OCI-based biomedical imaging are occurring at a rapid pace and we need to know where the limits are.

In the last part of my talk, I will introduce an effective, high-sensitive measuring platform that uses speckle as a source of information (rather than noise) for the assessment of the mechanical and dynamical properties of tissue and cells. This novel optical approach is relatively simple and yet powerful, and has the capacity to impact on global public health problems, such as malaria in resource-limited settings.

February 27

John Wahr, University of Colorado
Host:
Title: The GRACE satellite mission: using time-variable gravity to study the Earth

Abstract: NASA, in collaboration with the German Space Agency (DLR), launched the GRACE satellite gravity mission in the spring of 2002. The mission lifetime is presently projected to last through 2013. GRACE provides highly accurate monthly solution for the Earth's global gravity field. Differences between these solutions give information about time-variability in the gravity field, and so about month-to-month fluctuations in the Earth's mass distribution. The data are providing information about ice mass loss in Antarctica and Greenland, changes in the storage of water and snow on land, changes in ocean bottom pressure and ocean mass variability, and processes in the solid Earth (e.g post-glacial-rebound; very large earthquakes). This talk will describe the GRACE mission, and will include a representative cross-section of the kinds of things people are doing with GRACE data.

March 5

Edwin Williams, NIST Gaithersburg
Host: John Cumalat
Title: Experimental progress toward an SI, metric system, based on fundamental constants

Abstract: The NIST watt balance experiment is one of several systems that will provide a value for the Planck constant h and the elementary charge e to be used when the international General Conference on Weights and Measures, CGPM, approves an SI based primarily on scaling the SI so that c, e, h, k, and N_A have fixed values. The present relative standard uncertainty on h from the NIST experiment is 3.6 10^-8. Work is continuing with the target of improving our result by about a factor of two. Consistency with other determinations of h will likely be the key factor in deciding to proceed with the SI redefinitions in the year 2011.

March 12

Valerie Otero, University of Colorado School of Education
Host: Noah Finkelstein
Title: A Crisis in Physics Education: When Local Solutions Hit the National Scene

Abstract: The National Academy of Science listed four priority recommendations for ensuring American competitiveness in the 21st century. The first priority was to "Increase America's talent pool by vastly improving K-12 science and mathematics education" [1]. Studies point to content knowledge as one of the main factors that is positively correlated with teacher quality, yet those directly responsible for undergraduate physics content are rarely directly involved in elementary or secondary school-level teacher preparation. The Learning Assistant (LA) Model at the University of Colorado at Boulder is based on the premise that teacher preparation begins in the college of arts and sciences, where students begin their content education. The overarching goals of the LA program will be discussed with a focus on the positive impact that the Colorado Learning Assistant model has had on undergraduate physics students' content knowledge, on physics teacher production rates, and on the practices of other physics departments who are currently replicating our LA model. Our studies demonstrate the important role that physics research faculty play in elementary and secondary physics education.

1. Committee on Prospering in the Global Economy of the 21st Century, Rising Above the Gathering Storm (National Academy Press, Washington DC, 2006).

March 19

Charles Rosenblatt, Case Western Reserve University
Host: Ivan Smalyukh
Title: Optical Nanotomography of Anisotropic Fluids

Abstract: The physical properties of anisotropic fluids can be manipulated on very short length scales of 100 nm or less by appropriate treatment of the confining substrate(s). This facilitates the use of ordered fluids, such as liquid crystals, in a variety of applications ranging from displays to switchable optical elements such as gratings and lenses. Future advances will require a full understanding of the liquid crystal's structure at the nanoscale level. But owing to diffraction limitations, high resolution three dimensional imaging of the fluids's molecular orientation profile has been beyond the reach of extant optical techniques . Here we present a powerful new imaging approach based on the use of polarized light emitted from a tapered optical fiber to investigate molecular orientation in three dimensions at nanoscale levels. We immerse the fibers tip inside a thin layer of the fluid — in our case a nematic liquid crystal — that sits atop a substrate and raster-scan the fiber at a series of heights above the surface. From the images collected in the far field we are able to obtain a three dimensional visualization of the liquid crystal's structure with a resolvable volume nearly three orders of magnitude smaller than attainable by extant methods. We demonstrate this technique by examining a nematic liquid crystal whose director orientation is controlled by a nanoscopic pattern scribed into the underlying polymer-coated substrate. We are able to image the extrapolation length L ~ 200 nm over which the molecular orientation relaxes due to the liquid crystals elastic forces. This technique of acquisition and analysis of image slices offers the intriguing possibility of 3D nanoscale reconstruction of a variety of other soft materials.

March 26

Spring Break, no colloquium

April 2

Sidney Redner, Boston University
Host: Leo Radzihovsky
Title: Statistical Physics of Citations

Abstract: This talk will begin with basic empirical facts about the network of scientific citations, based on all Physical Review publications from the past 110 years. Intriguingly, the evolution of citations appears to be described by nearly linear preferential attachment. A master equation approach will be presented to characterize the citation distribution and related popularity-driven networks. The conditions that lead to exponential, power-law, or more singular citation distributions will be elucidated for general classes of networks. The citation data of Physical Review also reveals many interesting features that motivate new theoretical models. Finally, the Google page-rank analysis of citations will be applied to find "hidden gems" in Physical Review publications.

April 9

Piers Coleman, Rutgers University
Host: Michael Hermele
Title: Quantum criticality: black hole in the material phase diagram

Abstract: Quantum criticality: second-order phase transitions driven by quantum zero-point motion, have now been seen in a wide range of physical circumstance from magnets, ferro-electrics, to bilayer liquid Helium, heavy electron materials and possibly even high temperature superconductors. I shall talk about our current understanding of quantum criticality in metallic heavy electron systems, where a growing body of experimental evidence suggests that mainstay of our current understanding of electron matter, the electron quasiparticle concept, fails in a very serious fashion. What happens to the fabric of electronic matter at quantum critical point? Does quantum criticality nucleate high temperature superconductivity? Indeed, are spin fluctuations the "glue" of high temperature superconductivity - or do we need a still more radical view? These are questions I will touch upon in this colloquium.

April 16

Martin Goldman, University of Colorado
Host:
Title: Not all holes in space are black

Abstract: In the last few years isolated moving depressions ("holes") in the density of electrons have been measured in a wide variety of space and laboratory plasmas. These holes have been interpreted as nonlinear electrostatic states of the plasma in which electrons are trapped in self-consistent electrostatic potential wells. The bipolar electrostatic field associated with this state is often the signature of the moving hole. Three main questions are addressed in this talk:
          o How, where and when are these nonlinear hole states detected?
          o What is the simplest nonlinear analytic theory of stationary holes and how well does theory agree with measurement? (Goldman, et al, PRL Oct. '07)¹
          o What dynamical plasma processes might give rise to holes?
1. M. V. Goldman, D. L. Newman and A. Mangeney, PRL 99, 145002 (2007).

April 23

Dan Ralph, Cornell University
Host: Konrad Lehnert
Title: Dynamics and Spectroscopy in Nanoscale Magnetic Devices

Abstract: In the last ten years, a new mechanism has been discovered for manipulating the magnetic moment direction in small magnetic devices, as an alternative to applying magnetic fields. When a current of spin-polarized electrons interacts with a magnet, it can transfer spin angular momentum to the magnet and thereby apply a torque directly. This spin-transfer torque can be much more efficient than magnetic fields for switching magnetic elements in memory devices, and it can also drive interesting types of magnetic dynamics not easily generated by applied fields. I will describe the development of experiments to investigate the microscopic origin of this effect and to measure the ways in which individual nanomagnets respond to the spin torque, as well as potential applications. I will also discuss progress toward extending experiments on spin-dependent transport and magnetic dynamics from the 100 nm scale of the spin-torque devices to smaller samples. My group has helped to develop a technique for using electromigration to form mechanically-stable nanoscale contacts, for use in single-molecule studies. Even for simple bare contacts (containing no molecules) made of permalloy or nickel, we find an unexpectedly large changes in resistance as a function of the angle of the magnetization. We propose that this effect is the result of quantum interference of electrons within the magnetic device.

April 30 Special colloquium sponsored by the CU Energy Initiative and the Department of Mechanical Engineering

Mildred Dresselhaus, Massachusetts Institute of Technology

Title: Advanced Materials in our Energy Future: Breakthroughs and Challenges

Note special room: 4PM, Duane G1B30.

May 7 Note special room: 4 PM, JILA Auditorium

John Bohn, University of Colorado
Host:
Title: Better Chemistry Through Dipoles

Abstract: Don't let the title fool you. This talk is about the physics of interactions and collisions between molecules in ultracold gases. If the molecules possess a permanent electric dipole moment and are moving very slowly, then their behavior upon colliding can be manipulated to a surprising degree. I will explain how this control emerges from the combined effect of the electrostatic forces between the molecules and that of an applied electric field, aided by a little quantum mechanics. As an important potential application, I will also discuss the ability to make the molecules initiate a chemical reaction "on demand."

Colloquium schedules from previous semesters can be found here

Contact:

Michael Hermele