Physics Colloquia - 2009

Friday, 11 December 2009, 3:00pm, in Physics 135.

Detecting Alterations in Cell Ultrastructure with Optical Imaging: Implications for Cancer Screening

Dr. Vadim Backman, Professor, Biomedical Engineering Department, McCormick School of Engineering & Applied Sciences -- Northwestern University

Understanding cell functioning at the nanoscale has been hampered in part by the diffraction limited resolution of optical microscopy. We developed an optical microscopic spectroscopy technique, partial wave spectroscopy (PWS), that is capable of quantifying statistical properties of cell structure at the nanoscale. Our animal and human studies demonstrated that alterations in the nanoscale cell architecture is one of the earliest events in carcinogenesis and precedes any other known morphological changes at large length scales (i.e. microarchitecture). This appears to be a general event in carcinogenesis, which is supported by our data in three types of cancer: colon, pancreatic and lung cancers.

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.

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


(Tea and Cookies at 2:45pm)

Friday, 16 January 2009, 3:00pm

David Champion, Australia Telescope National Facility - CSIRO
The Discovery of An Unusual Pulsar Which Challenges Pulsar Evolution Theories
Location: Physics 135

The evolution of binary systems is governed by their orbital properties and the stellar density of the local environment. Studies of neutron stars in binary star systems offer unique insights into both these issues. In an Arecibo survey of the Galactic disk, we have found PSR J1903+0327, a pulsar with a 2.15-ms rotation period, in a 95-day orbit around a massive companion. The spin properties of this type of pulsar would normally be accounted for by a prolonged accretion history however the orbital properties, in addition to infra-red observations of the binary companion, suggest that accretion could not have occurred. Alternative formation scenarios include forming the system in a globular cluster then ejecting it into the Galactic disk, or membership in a hierarchical triple system.

Thursday, 22 January 2009, 3:00pm

Dr. Dali Liu, Brandeis University
Deciphering the Dynamics of Enzymatic Reactions through Crystallographic Snapshots
Location: Physics 135

While a crystallographic structure contains invaluable information on a protein, it is only considered a static snapshot taken during a dynamic biochemical process in which the protein participates. To take advantage of protein crystallography as a tool in mechanistic studies, we employed an experimental approach in which a series of crystallographic snapshots representing different stages of an enzymatic reaction can be obtained. Two successful stories, one on the inhibition of PLP-enzymes by a potential drug and the other on the catalysis of a bimetallo hydrolase, are presented. In each story, a series of high-resolution crystallographic structures were obtained via systematic variation of crystallization conditions. Crucial chemical species and their interactions with key residues in the active site were identified through careful interpretation of the electron density. Overall mechanisms of the enzymatic reactions were eventually constructed by compiling the series of crystallographic snapshots.

Friday, 30 January 2009, 3:00pm

David Kaplan, U C - Santa Barbara
Nearby, Thermally Emitting Neutron Stars: Laboratories for Extreme Physics
Location: Physics 135

Neutron stars are among the densest objects in the universe. The conditions in their centers are largely unconstrained by current theoretical physics or terrestrial laboratories, leaving a wide variety of compositions and structures possible. Observations of thermal emission from neutron stars -- specifically measurements of their sizes and cooling rates -- may therefore be the best way to constrain the behavior of matter in these extreme conditions. I will discuss a sample of nearby, cooling neutron stars that we are using for this purpose. We are attempting to pin down the basic parameters of these neutron stars with a variety of ground- and space-based observations, coupled with theoretical modeling. Along the way, we have encountered a number of interesting astrophysical puzzles that I will describe.

Friday, 6 February 2009, 3:00pm

Mark Wyman, Perimeter Institute
Beyond Vanilla Cosmology: The Search for Deviations from Inflationary Lambda-CDM
Location: Physics 135

Cosmology has reached a crossroads. A decade of high-quality data has established a cosmological standard model. At present, six well-measured parameters can explain many disparate observations. But the theory behind this model is far from clear. Our current model, successful though it is, is not firmly tied to fundamental physics. Completing it with an underlying theory may require some ingredients now considered exotic. In particular, I will also show that gravity itself is different if there are more than 3 large spatial dimensions, and suggest that the cosmological constant has its current small value because we live on a 3 dimensional membrane within the higher dimensional space. This model makes testable predictions for the history of cosmological structure formation; if it is the correct theory, it is possible that we could prove it in a few years' time!

Friday, 13 February 2009, 3:00pm

Tom Giblin, Bates College
Hawking Stars: Primordial Black Holes and Cosmology
Location: Physics 135

Some models of the early Universe predict the existence of black holes, Primordial Black Holes (PBH), just after the end of inflation. In this talk I will examine some consequences of having these objects present, and show how they modify the expansion history of the Universe. I will also present results predicting an observational signature of these short lived, ancient objects and what they can tell us about Cosmology.

Friday, 27 February 2009, 3:00pm

Liviu Movileanu, Syracuse University
Single-Molecule Biophysics with a Protein Nanopore
Location: Physics 135

Advances in rational protein design and single-molecule technology allow for biochemical sampling at high temporal and spatial resolution and for the detection, manipulation, and exploration of individual molecules. We have developed a methodology for examining single biopolymer dynamics within a protein nanopore, a simple system that is highly pertinent to several more complex biological processes such as the translocation of nucleic acids and polypeptides through transmembrane pores. The ionic current through a single protein nanopore was determined by single-channel electrical recordings in lipid bilayers. The results revealed unprecedented details of biopolymer behavior at single-molecule resolution. These examples demonstrate an unusual control of single biomolecules and pore-based nanostructures by using simple principles learn from modern biology.

Friday, 6 March 2009, 3:00pm

Shobhana Narasimhan, Jawaharlal Nehru Centre for Advanced Scientific Research (Bangalore, India)
Ab Initio Calculations on Surfaces and Clusters
Location: Physics 135

In the past few decades, ab initio density functional theory (DFT) has emerged as a powerful technique for calculating the properties of materials from first principles (i.e., with no empirical input apart from atomic numbers and atomic masses). I will describe some attempts in our group to use this technique to gain an understanding of the properties of low-dimensional materials, as well as to use this understanding to design new materials. Some examples of possible uses for these materials include novel catalysts for pollution control, and new magnetic materials for information storage. I will also present some calculations to see how the properties of very small objects (consisting of tens of atoms) vary as a function of size.

Tuesday, 10 March 2009, 3:00pm

Dawn Erb, UC-Santa Barbara
Galaxies in the Young Universe: Kinematics, Elemental Abundances and Gas Flows
Location: Physics 135

A large fraction of the stars in the universe today formed during the redshift interval 1.5<3, when the universe was only about 25% of its current age and galaxies were more disordered than those that surround us today. However, the quantitative study of galaxies in this redshift range from large spectroscopic samples has only recently become feasible, due to advances in large telescopes and instrumentation. Such spectra offer a unique opportunity to quantify the physical conditions in these distant galaxies and their interactions with the surrounding intergalactic gas. I will discuss the results of a large near-IR spectroscopic survey of star-forming galaxies at z~2, highlighting physical inferences regarding the overall properties of the galaxies, their internal structure, elemental abundances, and large-scale outflows and inflows of gas.

Friday, 13 March 2009, 3:00pm

Antonio Delgado, Asst. Professor, Theoretical High Energy Physics, Univ. of Notre Dame
Unparticle Physics
Location: Physics 135

In 2007, Howard Georgi introduced the concept of "unparticle" states that are described by a conformal theory. In this colloquium I will explain what a conformal theory is, what exactly an unparticle is, and the interesting experimental consequences that they may have.

Friday, 27 March 2009, 3:00pm

Professor Humphrey Maris, Dept. of Physics, Brown University
Experiments with Electrons in Superfluid Helium
Location: Physics 135

I will describe experiments we have performed in which we are able to image the motion of individual electrons moving in liquid helium-4. Electrons in helium form bubbles of radius ~ 19 m. We use the negative pressure produced by a sound wave to expand these bubbles to a radius of about 10 m. The bubbles are then illuminated with light from a flash lamp and their position recorded. We report on several interesting phenomena that have been observed in these experiments. It appears that the majority of the electrons that we detect result from cosmic rays passing through the experimental cell. We discuss this mechanism for electron production.

Friday, 3 April 2009, 3:00pm

Dr. John Beacom, Assoc. Professor, Depts. of Physics and Astronomy, Ohio State University
New Vistas in Astronomy Above 1 TeV (1.6 erg) per Particle
Location: Physics 135

The large measured fluxes of high-energy but non-directional cosmic rays tell us that astrophysical sources capable of efficiently accelerating charged particles must exist. While this has been known for decades, just what these sources are remains mysterious. Gamma-ray observatories have recently discovered dozens of Milky Way sources, many of which are as luminous in TeV gamma rays as the Sun is in optical photons. I will discuss the exciting prospects for understanding these objects and the consequences of their existence. Neutrinos will be a decisive probe of how these cosmic accelerators work.

Friday, 10 April 2009, 3:00pm

Paul Selvin, Professor of Physics & Biophysics & Professor Affiliate of Cell & Developmental Biology , Univ. of Illinois - Urbana Champaign
FIONA Looks at Individual Molecular Motors Walk and Run
Location: Physics 135

The standard diffraction limit of light is about 250 nm, meaning that you cannot "resolve" objects closer than this distance. Despite this, we have come up with a method to measure 1.5 nm in x-y plane, with 1-500 msec, using a technique we call Fluorescence Imaging with One Nanometer Accuracy (FIONA). FIONA also has the advantage that it looks at individual, or single, proteins. We have chosen to study molecular motors, both in vitro and in vivo. We find that all tested motors walk in a hand-over-hand fashion, in vitro and in vivo. We also find evidence that in vivo, two of the same motors carry cargo simultaneously-but not cooperatively. We also see passing of cargo from one type of motor to another. Finally, we find that, in vivo, things walk around a microtubule, not just co-linear with them.

Friday, 17 April 2009, 3:00pm

Professor E. V. Sampath Kumaran, Tata Institute of Fundamental Research, Mumbai, India
Puzzles in the Electron Transport Behavior of Rare-earth Intermetallics
Location: Physics 135

The field of Strongly Correlated Electron Systems is a few decades old, but there remain difficulties in (even) the qualitative understanding of physical properties of such materials. After reviewing this situation for Ce- systems, I will bring out that even the so-called "normal" rare-earth systems present unusual situations with respect to electron transport, raising interesting new questions in magnetism.

Friday, 24 April 2009, 3:00pm

Dr. Larry Jackel, Rutgers University
Autonomous Ground Robots: Will you be riding one?
Location: Physics 135
TBA

Friday, 1 May 2009, 3:00pm

Dr. Stefano Marchesini, Advanced Light Source, Lawrence Berkeley National Laboratory
Imaging with X-Ray Lasers
Location: Physics 135

Ever since Wilhelm Röntgen photographed his wife's hand in 1896, the imaging power of X-rays has been exploited to help see the unseen. The penetrating power of x-rays allows us to view the internal structure of many objects, while their short wavelength allows scientists to look at nanometre-scale objects. Achieving near-atomic spatial resolution, however, is still a huge challenge. The high penetration of x-rays makes the production of high-resolution lenses difficult, and high doses destroy live samples beyond recognition before atomic resolution can be achieved. Recent technological breakthroughs promise to solve these problems for the first time. The development of free-electron lasers now offer the realistic prospect of imaging on the time-scale of atomic motion. So bright is the flash of x-ray light that a sample is vaporized, but not before an image or a hologram is recorded. Even so, building lenses capable of atomic resolution is still well beyond reach. The development of lensless ("diffractive") X-ray imaging techniques appears to cut this gordian knot by replacing the necessary lens with a computer algorithm to recombine the scattered photons, providing an image whose resolution is limited in principle only by the X-ray wavelength. This combination of brighter sources, computer algorithms and improved detectors promises a giant leap in the structural characterization required for developing nanoscience and nanotechnology.

Friday, 8 May 2009, 3:00pm

Gabe Shaughnessy, Postdoctoral Associate, Northwestern University and Argonne National Laboratory
The Nature of Dark Matter
Location: Physics 135

The nature of dark matter has been elusive. However, many exciting experimental efforts have aimed to observe it directly via nuclear recoil, or indirectly via its annihilation products. Indeed, recent results from the PAMELA satellite show an excess is high energy positrons which may be a signature of dark matter annihilations. I will discuss the prospects for observing dark matter indirectly via high energy neutrinos, positrons and gamma rays over a variety of models.

Friday, 15 May 2009, 3:00pm

Christian D. Ott, TAPIR, Caltech
The Death of Massive Stars: Core-Collapse Supernova Mechanisms and Their Signatures in Gravitational Waves
Location: Physics 135

Despite many decades of concerted theoretical effort and numerical modeling, the details of the core-collapse supernova explosion mechanism are still under debate. Indications are strong that the supernova mechanism is intrinsically multi-dimensional and involves (a combination of) postbounce energy deposition by neutrinos, convective instability, the standing-accretion-shock instability (SASI), protoneutron star core oscillations, rotation, and magneto-hydrodynamic effects. I review the current status of core-collapse supernova modeling and introduce the ensemble of candidate explosion mechanisms that is emerging from recent multi-dimensional simulations. I go on to discuss gravitational-wave (GW) emission processes in core-collapse supernovae and present new results on the supernova gravitational-wave signature that were obtained with 2D/3D general relativistic and Newtonian simulations. I demonstrate how GWs observed by current and future LCGT/advLIGO-class detectors could be used to constrain the core-collapse supernova explosion mechanism.

Friday, 22 May 2009, 3:00pm

Dr. Brynmor Davis, Postdoctoral Research Associate, UIUC, Dept. of Bioengineering
Computational Microscopy: Using Modeling and Inference to Improve Optical Sensing
Location: Physics 135

Most familiar optical imaging systems produce data that can be immediately associated with object structure, i.e., an image is measured directly. However, practicable imaging systems need not be restricted to those that produce a direct object-to-data relationship. Physical structure and other object features may be encoded in data that form no obvious image; by solving a model-based inverse problem, this information may be recovered. Including physically-motivated data processing in the sensing system thus allows the relaxation of standard instrument design constraints. Freeing instrumentation from the restrictions of direct image formation opens the door to new and optimized measurement schemes. The computational approach to imaging has been applied, with great success, in techniques such as magnetic resonance imaging, computed x-ray tomography and synthetic aperture radar. In this talk, examples of computational optical microscopy systems and applications will be described. Applications include probing the conformation of DNA, characterizing scattering from nanoparticles and three-dimensional imaging of the scattering properties of tissue. The need for computational imaging approaches in mid-infrared absorption microspectroscopy will also be discussed.

Thursday, 9 July 2009, 3:00pm

Seth King, Ph.D. Candidate, University of Wisconsin-Milwaukee
Investigation of Structure and Growth on Polar Surfaces of Wide-Bandgap Semiconductors
Location: Physics 135

Low energy electron diffraction revealed a previously unreported (3x3)R30 reconstruction (Fig. 1) on clean, O-polar ZnO surfaces after in-situ preparation under conditions with an extremely low H background. It has been proposed that an unreconstructed, H-free, O-polar ZnO surface cannot be produced, and that residual H may serve to stabilize the commonly observed (1x1). Interestingly, an ex situ tube-furnace annealing procedure, commonly used to create uniform ZnO surfaces, was also found to produce a (3x3)R30 reconstruction, similar to what has been observed with other oxide surfaces. As the sample is prepared from the as-received surface, to a clean (1x1) termination, and finally to the clean (3x3)R30 reconstruction, x-ray photoelectron spectroscopy shows decreasing intensity of the hydroxyl shoulder located to the high-binding-energy side of the O1s peak. We find that this reconstruction is stable against H2, N2, and air, although its formation is suppressed when preparation occurs under an intentional H2 background. Observing a (3x3)R30 reconstruction on ZnO(000-1) surfaces after very different preparation conditions leads to an important question: Do these reconstructions have the same structure? Even in absence of a structural model, this question may be readily answered by employing LEED-IV analysis. The calculated Pendry R-factor comparing LEED-IV curves taken from both vacuum and tube furnace prepared samples is 0.27, which suggests the reconstructions share a common structure. These data were also fit to deduce a possible structural model for the reconstructed surface. Our results support the possibility of a H-stabilized ZnO(000-1)-(1x1) surface, and suggest that caution must be taken when tube-furnace annealing is used to prepare ZnO substrates, as subsequent growth on reconstructed surfaces may behave differently from that on the assumed (1x1).

Thursday, 23 July 2009, 3:00pm

Swapnil Tripathi, Ph.D. Candidate, University of Wisconsin-Milwaukee
Self-Force on Point Particles in Curved Spacetime and Quadrupole Moments of Rotating Neutron Stars
Location: Physics 135

In this talk I will discuss two topics relevant to the area of gravitation wave detection. First, I will talk about self-force on point particles in curved spacetime. In a curved background the self field of the particle acts back on itself to push it off the geodesic of the background spacetime. This effect is due to interaction of the charge or mass with its own field, which produces self-force. The self force contains both conservative and dissipative terms, and the latter can be interpreted as a radiation reaction force. I will discuss the earlier work done on self force and my own work in which I have calculated the self force on a scalar particle in curved spacetime. This work has application to calculating gravitational wave-forms radiated for EMRI (Extreme Mass Ratio Inspirals). In the second part of my talk I will discuss computation of quadrupole moment (Q) of rotating neutron stars (RNS). In a binary inspiral of rapidly rotating stars, the quadrupole moment of the stars dominates the departure from a point-particle wave form. I will talk about setting upper limit on quadrupole moment of RNS consistent with causality. I will also talk about correcting an error in previous works on computations of Q and finally give an empirical relation for calculating Q of slowly rotating neutron stars.

Friday, 25 September 2009, 3:00pm

Mark Trodden, Prof. of Physics and Astronomy and Co-Director of the Center for Particle Cosmology, Univ. of Pennsylvania
Is Cosmic Acceleration Telling Us Something About Gravity?
Location: Physics 135

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.

Friday, 16 October 2009, 3:00pm

James Wittig, Vanderbilt University
Characterization of Magnetic Recording Media and High Anisotropy FePt Magnetic Nanoparticles
Location: Physics 135

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, 30 October 2009, 3:00pm

Professor Glennys Farrar, Dept. of Physics and Director of the Center for Cosmology & Particle Physics, NYU
TBA
Location: Physics 135
TBA

Friday, 6 November 2009, 3:00pm

Professor Simon Billinge, Applied Physics & Applied Mathematics (Columbia University) and Condensed Matter and Materials Science (Brookhaven National Laboratory)
The Nanostructure Problem in Materials Science
Location: Physics 135

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, 20 November 2009, 3:00pm

George N. Phillips, Jr., Professor of Biochemistry and Computer Sciences , Dept. of Biochemistry, Univ. of Wisconsin-Madison
TBA
Location: Physics 135
TBA