Fall 2009 Colloquia
Friday, 20 November 2009, 3:00pm, in Physics 135.
Dynamics of Proteins in Crystals -- Beyond Static Snapshots
Dr. George N. Phillips, Jr., Professor of Biochemistry and Computer Sciences , Dept. of Biochemistry, Univ. of Wisconsin-Madison
Proteins molecules, even the crystalline state, are variable in conformation and can undergo dynamic transitions. Methods for studying these dynamics will be reviewed, including sub-nanosecond time resolved X-ray crystallography.
Friday, 13 November 2009, 3:00pm, in Physics 135.
Ultrafast electron microscopy: Problems and solutions
Professor W. Andreas Schroeder, Department of Physics, University of Illinois - Chicago
Ultrafast electron microscopy (UEM) is a technique that aims to combine the high spatial (sub-nanometer) resolution of electron microscopy with the high temporal resolution (sub-picosecond) afforded by today’s ultrashort pulse laser systems. To date, imaging dynamic transmission electron microscopy (DTEM) has realized a space-time resolution of close to 10nm.ns in a single-shot regime and about 1.nm.ps at ~100MHz (multi-shot) repetition rates, albeit with only a single electron per pulse. The performance of both of these DTEMs has been shown to be limited by space-charge effects; in the spatial dimension for the former ~10ns-pulsed instruments and predominantly in the temporal dimension for the latter UEM driven by a femtosecond laser radiation source.
In this talk, I will describe the two effects of electron-electron coulomb scattering: (i) global space-charge effects that influence the shape dynamics of the electron pulse, and (ii) stochastic collisions that destroy any coherence (or information) in the electron pulse. The efficient delivery of the electron pulse to the specimen is determined mainly by global space-charge effects that can be minimized and compensated for through the appropriate choice of initial three-dimensional electron pulse shape and perhaps laser-driven photoemission process, combined with the use of magnetic electron lenses and RF pulse compression cavities. The latter stochastic electron-electron scattering, which primarily affects image quality, is a more complex post-specimen problem that may restrict the performance of future UEMs.
Friday, 6 November 2009, 3:00pm, in Physics 135.
The Nanostructure Problem in Materials Science
Professor Simon Billinge, Applied Physics & Applied Mathematics (Columbia University) and Condensed Matter and Materials Science (Brookhaven National Laboratory)
A diverse array of complex materials and structures are driving the nanotechnology and molecular biology revolutions. To understand and design these materials, it is essential to perform high precision structural characterization at the nanoscale. Often, even sub-Angstrom changes in inter-atomic bond lengths have profound consequences for the chemistry and functionality of these structure-sensitive materials. Crystallographic methods are the gold standard for atomic structure determination, however a broad and growing class of materials and/or nanophase morphologies do not yield to a crystallographic analysis. The scattering is diffuse and Bragg-peaks become broad and overlapped. This is "the nanostructure problem" which currently has no robust solution. I will discuss alternative, more broadly applicable, methods that make use of advanced x-ray and neutron scattering sources, which are emerging for these nanostructure problems, and give some examples of their application.
Friday, 30 October 2009, 3:00pm, in Physics 135.
Ultrahigh Energy Cosmic Rays: experimental progress and the puzzles it presents
Professor Glennys Farrar, Dept. of Physics and Director of the Center for Cosmology & Particle Physics, NYU
I will survey the field of ultrahigh energy cosmic rays (UHECRs), report on recent observational developments, and describe the theoretical puzzles presented by the data. UHECRs are the highest energy particles in nature -- some having more than 10^8 times higher energy than the LHC beam achieves. Learning how nature produces them will surely reveal some fascinating astrophysics, and exploiting them for particle physics can provide a glimpse of particle interactions at energies beyond the reach of accelerators.
Friday, 23 October 2009, 3:00pm, in Physics 135.
Photophysics of polyacenes: from exciton delocalization to exciton fission
Assoc. Professor Christopher Bardeen, Dept. of Chemistry UC-Riverside
Organic semiconductors are promising materials for a variety of applications, including field-effect transistors and photovoltaic cells. The observation of exciton fission, where a singlet exciton spontaneously splits into two triplet excitons, provides the motivation for studying polyacene materials (such as molecular crystals composed of anthracene and tetracene) as a way to enhance photovoltaic efficiency. Examination of both covalent tetracene dimer molecules and the crystals indicates that wavefunctions delocalized over several molecules are required to facilitate fission.
Using femtosecond time-resolved spectroscopy methods, we examine the dynamics of these states and compare them with the excited state dynamics of isolated molecules. Practical ways to make nanoscale molecular crystals with uniform size and shape that could be incorporated into actual organic photovoltaic devices will also be discussed.
Friday, 16 October 2009, 3:00pm, in Physics 135.Friday, 16 October 2009, 3:00pm, in Physics 135.
Characterization of Magnetic Recording Media and High Anisotropy FePt Magnetic Nanoparticles
James Wittig, Vanderbilt University
Areal densities in magnetic recording have exhibited Moore's Law like increases. This is partially due to improvements in the media microstructure with reduced grain sizes, tighter grain size distribution, and chemical isolation between grains. With the recent shift from longitudinal to perpendicular recording, areal densities have again continued to increase with demonstrations of over 400 Gbits/in2. However, areal density is limited by thermal stability considerations where the ratio of stored magnetic energy KuV (anisotropy energy times the magnetic switching volume) to the thermal energy kT must be ~ 50-70. The projected limit for traditional CoPtCr(X) granular media is on the order of 500 Gbits/in2. Further increases in the areal density will require greater reduction in the grain size (switching volume), which necessitates finding media with higher anisotropy to maintain thermal stability. Possible candidate materials systems include FePt and SmCo5, which have bulk Ku values 50 to 100 times greater than CoPtCr(X) media materials. High Ku allows for thermally stable grains sizes down to ~ 2.5 nm, which would permit areal densities in the Tbit/in2 regime. Monodispersed FePt nanoparticles (diameter 3-10 nm) produced by chemical synthesis have great potential for future magnetic storage media. The as-synthesized FePt nanoparticles are face-centered cubic (FCC) and require annealing to chemically order into the tetragonal L10 structure (CuAuI). Understanding the L10-ordering phase transformation is critical for using these FePt nanoparticles as engineered magnetic nanostructures. The current study employs high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), also known as atomic-number contrast or Z-STEM. Using the Oak Ridge National Laboratory (ORNL) aberration corrected JEOL 2200FS-AC, STEM imaging was performed at 200 kV with a sub-0.1-nm probe to investigate the development of L10 order in individual FePt nanoparticles. Complementary chemical composition data was obtained with nanoprobe energy dispersive spectroscopy (EDS) of individual particles using a Philips CM200-FEG. A combination of ex-situ and in-situ studies of the FCC to L10 phase transformation will be presented.
Friday, 9 October 2009, 3:00pm, in Physics 135.
Benefits of Aberration Corrected TEM for Material Science Problems
Bernd Kabius , Center for Electron Microscopy, Materials Science Division, Argonne National
During the last 10 years, several aberration-correction concepts for electron microscopes have succeeded in improving spatial resolution and analytical capabilities. Electron optical systems for correction of spherical aberration are now a valuable tool for material science research and several investigations have already exploited some of the benefits of Cs-correction for high-resolution TEM and STEM. The TEAM project is a collaborative DOE project which will extend the present capabilities of aberration correction technology. The goals for aberration correction within the TEAM project are: Correction of higher order aberrations such as fifth order spherical aberration is required for improving interpretability at sub-Angstrom resolution (TEM) and higher beam currents in smaller electron probes (STEM); and, improving the information limit to 0.5Å by correction of chromatic aberration (Cc) and energy monochromation. This progress in electron beam instrumentation is expected to have a strong impact on in-situ TEM, magnetic imaging and analytical electron microscopy. The benefits of Cs - and Cc - correction for ma-terial science problems requiring these methods will be discussed and first results using Cc-correction will be presented.
Friday, 25 September 2009, 3:00pm, in Physics 135.
Is Cosmic Acceleration Telling Us Something About Gravity?
Mark Trodden, Prof. of Physics and Astronomy and Co-Director of the Center for Particle Cosmology, Univ. of Pennsylvania
Among the possible explanations for the observed acceleration of the universe, perhaps the boldest is the idea that new gravitational physics might be the culprit. In this colloquium, I will discuss some of the challenges of constructing a sensible phenomenological extension of General Relativity, give examples of some candidate models of modified gravity and survey existing observational constraints on this approach. I will conclude by discussing how we might hope to distinguish between modifications of General Relativity and dark energy as competing hypotheses to explain cosmic acceleration