Colloquia Archive: 2010

Monday, 20 December, 3:00 PM. Physics 135

The Handwaver's Guide to Dark Matter Halos

Neal Dalal, Canadian Institute for Theoretical Astrophysics (Toronto)

Dark matter halos are the endpoints of cosmological structure formation. They play a crucial role in many areas of astrophysics and cosmology. Our understanding of halos is based almost entirely on numerical experiments in N-body simulations, with relatively poor theoretical understanding of what determines halo properties. In my talk, I will try to give a simple way to understand many properties of halos, including their density profiles, their abundance, and their clustering.

Using an extremely simple model, it is possible to match the results of N-body simulations across a wide range of cosmologies, even better than commonly used empirical fitting formulae. Using this approach, we can also predict how halos and their resident galaxies behave in cosmologies that depart from the standard LCDM model, which I will illustrate with some examples.

Friday, 17 December, 3:00 PM. Physics 135

The Electromagnetic Precursor of Binary Black Hole Mergers

Philip Chang, Canadian Institute for Theoretical Astrophysics (Toronto)

Galaxy mergers, which are a natural consequence of hierarchical assembly of galaxies, are expected to produce binary black holes, which subsequently merge. The detection and analysis of gravitational waves from these sources is the major aim of the next generation gravitational wave detector: LISA, the Laser Interferometric Space Antenna.  These gravitational waves encode a tremendous amount of information, but to make the connection with astrophysics and cosmology, it is necessary to identify the galaxies hosting these mergers via the associated electromagnetic counterpart to these mergers.

I will describe these mergers events and discuss the various regimes where potential electromagnetic counterparts can be found.  I will also describe some recent work, which holds much promise for the prompt identification of these mergers — an electromagnetic precursor from tidal forcing.

Thursday, 16 December, 1:30 PM. Physics 137

Gamma-ray probes of new physics: Particle dark matter and intergalactic magnetic fields

Shin’ichiro Ando, Caltech

One of the important goals for these telescopes is to study the nature of dark matter, by looking for annihilation gamma rays.  Because dark matter is annihilating everywhere in the Universe, the diffuse gamma-ray background will contain some information on dark matter.  In this talk, I introduce a new analysis method of the gamma-ray background, by using the angular power spectrum.  This way, one can efficiently analyze spatial distributions of photons, and since dark matter annihilation follows density in a different way from other astrophysical sources, the angular power spectrum is also characteristic and distinguishable.  The dark matter can be identified in the angular power spectrum with Fermi if its contribution is ~10% or more.

Another important physics that one can learn with gamma rays is magnetic fields in intergalactic space.  This is because distribution of secondary photons from electromagnetic cascades is affected by magnetic fields.  We therefore analyzed stacked AGN images from Fermi data, and found an anomalous excess compared with point-spread function.  If this is due to intergalactic magnetic fields, then the field value will be around 1 femto-Gauss.

Wednesday, 15 December, 3:00 PM. Physics 135

Cosmology from 17,000 Feet: Results from the Atacama Cosmology Telescope

Jonathan Sievers, Canadian Institute for Theoretical Astrophysics (Toronto)

The Atacama Cosmology Telescope (ACT) is observing the Cosmic Microwave Background (CMB) from high in the Chilean Andes. The CMB provides a snapshot of the universe when it was only 400,000 years old, well before any stars or galaxies had formed. The nearly uniform density (to a few parts per hundred thousand) means that well understood linear physics describes the physics of the CMB and lets us transform observations into constraints on fundamental parameters of the cosmos in a straightforward manner. ACT provides a significant improvement to our knowledge of the CMB on small (few arcminute) scales.

We present the results from the first full season of ACT data and what we learn from them, including better limits on the number of relativistic species, initial power spectrum from inflation, early helium density, potential contributions to structure formation from cosmic strings, and the imprint of galaxy clusters on the CMB.

Friday, 10 December, 3:00 PM. Physics 135

Structure and Dynamics from Ultralow-Signal Random Sightings

Peter Schwander, UW-Milwaukee

At least in biology, it is well-known that structure determines function, and there is increasing evidence that structure is neither immutable, nor static. The study of structural variability and dynamics represents a crucial but difficult frontier in biology and soft condensed matter physics.

I describe how advanced manifold-mapping techniques, augmented with concepts from Riemannian geometry and general relativity, can be used to determine the structure and dynamics of evolving systems from a random collection of ultra-low-signal snapshots emanating from unknown orientations and conformations.   

Friday, 3 December, 3:00 PM. Physics 135

Understanding Reionization of the Universe: Resolved Images of Ionizing Emission from High-Redshift Galaxies

Brian Siana, Caltech

In order for star-forming galaxies to reionize the intergalactic medium (IGM) at z > 6, a significant fraction of the ionizing photons must escape into the IGM. Currently, this ionizing photon “escape fraction”  is poorly constrained, and it is not understood how large escape fractions are possible in star-forming galaxies.  

I will discuss several programs using the Hubble Space Telescope to directly detect the ionizing emission from galaxies in the early universe.  With these data we can determine how ionizing photons escape to better understand how star-forming galaxies reionized the universe.

Friday, 12 November, 3:00 PM. Physics 135

Cosmic Ray Anisotropy Measurement with IceCube

Rasha Abbasi, UW-Madison

IceCube is a 1 kilometer-cubed, neutrino observatory, located at the geo-graphical South Pole. The kilometer-cubed detector construction is on schedule to be completed in 2011. At the moment, it is taking data with 79 deployed strings; when completed, it will comprise 80 strings, plus 6 additional strings for the low energy array Deep Core. The strings are deployed in the deep ice at a depth between 1,450 and 2,450 meters, each string containing 60 optical sensors.

In this talk I will present selected results of on-going analysis of the IceCube detector data, including the search reporting the measurement of the cosmic ray anisotropy. The data used in the anisotropy analysis contains billions of downward moving muon events with a median energy per nucleon of ~20 TeV and a median angular resolution of 3 degrees. The energy dependence of this anisotropy is also presented. The observed anisotropy has an unknown origin and we will discuss various possible explanations.

Studies of the anisotropy could further enhance the understanding of the structure of the galactic magnetic field and possible cosmic ray sources.

Wednesday, 3 November, 3:00 PM. Physics 135

Theoretical study of singlet-triplet mixing in FFLO superconductors

Zhichao Zheng, UWM Physics PhD Candidate

In superconductors, Cooper pairs are composed of particles with spin 1/2. The spin component of a pair wave function can be characterized by its total spin S=0 (singlet) or S=1 (triplet). Among the possible superconducting phases in the presence of a strong magnetic field, a spatially nonuniform state proposed by Fulde and Ferrell as well as by Larkin and Ovchinnikov has become a subject of intense interest. For the singlet superconductor, a magnetic field tends to break the pairs due to the Zeeman effect. Usual superconductors are spatially uniform, but in the FFLO state, this pair breaking is reduced by the formation of a new pairing state which spatially oscillates.

Friday, 22 October, 3:00 PM. Physics 152

Architecture of allosteric proteins: Crystallographic exploration of cooperativeinvertebrate hemoglobins

William Royer, Biochemistry and Molecular Pharmacology, UMASS Medical School

Assembly of protein complexes is an important strategy for regulating biological activity. Invertebrate hemoglobins provide a wealth of systems for investigating how assembly impacts function. In this talk, we will discuss issues of intersubunit communication, ligand migration and assembly for invertebrate hemoglobin assemblages ranging in size from a simple dimer to complex assemblages of 180 subunits. Our extensive conventional and ultra-fast time-resolved crystallographic analysis on the simplest of these hemoglobins, the homodimeric Scapharca HbI, reveals how the protein uses interface water molecules in the communication between subunits. Investigation of ligand migration through this protein suggests, surprisingly, that ligands primarily escape through a distal histidine gate at the subunit interface, which emphasizes the central role of dynamics in protein function.

The pairing of subunits with extensive contacts involving the heme-embedding E and F helices found in HbI is also observed among all other cooperative invertebrate hemoglobins whose crystal structures have been determined to date. We will discuss our preliminary time-resolved crystallographic studies on Scapharca tetrameric hemoglobin and our conventional crystallographic studies on much larger assemblies.

Thursday, 14 October, 1:30 PM. Physics 137

Tuberculosis: Rational Minds Will Prevail

Lee W. Tremblay, Department of Biochemistry, Albert Einstein College of Medicine, Bronx, NY

Mycobacterium tuberculosis is the causative bacterial agent of human tuberculosis (TB) and is at the root of a global health care crisis with 1.7 million deaths occurring in 2006 alone. The effectiveness of the current multi-drug, six-month chemotherapeutic treatment is being jeopardized by the emergence of strains of TB that are ineffective against TB, our recent report demonstrating that a combination of two FDA approved β-lactams, meropenem and clavulanate, work synergistically to sterilize 13 clinical isolates of XDR-TB has brought about a reexamination of this class of antibacterial agents. We have structurally determined the mechanisms by which these two drugs inhibit the genomically encoded TB β-lactamase (BlaC) utilizing X-ray crystallography. In addition we successfully engineered and crystallized mutant K73A-BlaC and E166A-BlaC proteins. These mutant enzymes are respectively unable to acylate and deacylate the β-lactam substrate within the active site, allowing us to trap chemical intermediates within the active site. We have implemented a systematic approach of X-ray crystallography to obtain high-resolution structures (<1.5 Å) of multiple β-lactam substrates within the active site of the mutant BlaC enzymes. 

The crystallographic data are revealing with atomic resolution the chemistry and interactions amongst various classes of β-lactam substrates within the BlaC active site. Substrates with relevant structural results are subjected to a thorough steady state kinetic analysis. The kinetic parameters allow us to quantitatively describe the substrate specificity and affinity of BlaC for each substrate. The structural and kinetic data provide information, allowing us to account for important chemical, electrostatic and steric factors, as well as steady state binding affinities. These variables are being incorporated into the rational design of lead compounds. These compounds are being synthesized through our collaboration with Courtney Aldrich at the Center for Drug Design at the University of Minnesota. We will then in turn, test these compounds for in vitro inhibition of BlaC and where successful further test those compounds for in vivo activity against TB.

Friday, 8 October, 3:00 PM. Physics 152

Atoms and Molecules in Ultra-Intense Laser Fields

Barry Walker, University of Delaware, Dept. of Physics and Astronomy

I will discuss the physics of atoms and molecules in “ultra” high intensity laser fields. Light whose intensity of 10^19 W/cm^2 is billions and billions of times more intense than sunlight on the earth. New and exciting excitation processes occur in these high fields where the particle-field interactions lead to relativistic motion and the laser magnetic field cannot be treated as approximately zero.

Our recent work will highlight (1) the observations of MeV photoelectrons from the laser ionization of atoms and (2) the role of “rescattering events” during ionization where photoelectrons are driven back into the parent ion by the ultra-strong laser field.

Wednesday, 6 October, 3:00 PM. Chemistry Bldg. Room 170

Merger of a Black Hole and a Neutron Star

Masura Shibata, Kyoto University, Japan

Coalescence of a compact binary composed of a black hole (BH) and a neutron star (NS) is one of the most promising sources of ground-based gravitational-wave detector such as LIGO, VIRGO, and LCGT, and also a possible progenitor of the central engine of short gamma ray bursts.

Numerical relativity is probably the unique approach for theoretically clarifying the late inspiral and merger processes and the final fate of the binary. We report our latest effort for the numerical relativity simulation of BH-NS binaries, focusing on gravitational waveforms and the final remnant. In particular, strong dependence of the gravitational waveforms and the final remnant on the equations of state of neutron star matter is emphasized.

Friday, 10 September, 3:00 PM. Physics 152

Dielectric Spectroscopy-Based Monitoring of the Cellular Response to Ligand Binding to Membrane Receptors In Vivo

Stephen A. Boppart, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Illinois

Nonlinear interferometric vibrational imaging (NIVI) uses the chemical sensitivity of coherent anti-Stokes Raman scattering (CARS) for endogenous vibrational contrast. By combining chirped-CARS with Fourier transform spectral interferometry, NIVI extracts the complex third order nonlinear susceptibility χ(3). This can retrieve the
Raman lineshape by eliminating the non-resonant background that plagues CARS. This presentation will introduce this technology and present proof-of-principle results identifying the potential of NIVI for breast cancer diagnostics.

Specifically, a diagnostic algorithm has been trained to predict the pathological state of rat breast tissue with < 1% classification error based on the spectral features captured by NIVI. Preliminary studies show substantial promise for the NIVI reconstructed tissue maps to have comparable value to stained histological images, with added information on the spatially-dependent molecular composition, potentially making it a sensitive clinical diagnostic tool for real-time stain-free or label-free histopathological images.

Friday, 3 September, 3:00 PM. Physics 152

Dielectric Spectroscopy-Based Monitoring of the Cellular Response to Ligand Binding to Membrane Receptors In Vivo

Michael Stoneman, UWM

When exposed to alternating electric fields, microscopically inhomogeneous materials are polarized by various mechanisms, and their dielectric properties present strong frequency dependence. In the technique of dielectric spectroscopy, an alternating electric field is applied to a sample, and the permittivity and conductivity of that sample is measured over a range of frequencies. The information uncovered using dielectric spectroscopy measurements can reveal both morphological and electrical properties of the constituents of the sample at various size scales.

Current methods for the monitoring of protein interactions in vivo rely on tagging the proteins of interest by fluorescent markers. However, it is ideal to be able to monitor protein activity without perturbing the system under study. Because of its non-invasive nature and ability to probe both the surface properties as well as the internal layers of biological systems without the need for tagging, dielectric spectroscopy has the potential to play a vital role in protein interaction studies.

In this talk, I will introduce the technique of dielectric spectroscopy and present its application to the study of living yeast cells expressing a well-known member of the G protein coupled receptor family, the Sterile2 α-factor receptor protein (ste2p). The radiofrequency dielectric properties of the various cellular regions were compared between populations of cells which were exposed to the natural binding ligand of ste2p, the α-factor, and those which were not. Interestingly, large changes in both the membrane region of the cells, where the ste2p/α-factor interaction occurs, as well as the cellular interior dielectric properties were detected after the addition of α-factor to the cell suspending medium. A number of different complementary experiments performed on the yeast will be presented in an effort to both support and help interpret these dielectric measurements. The results of this large scale study demonstrate the ability of dielectric spectroscopy to non-invasively monitor protein/peptide interactions as well as discern specific and non-specific cellular reactions to alterations in the cellular external medium.

Wednesday, 18 August 2010, 3:00 PM. Physics 152

Swift Heavy Ion Induced Conducting Track Formation in Fullerene (C60and C 70) Thin Films

Ambuj Tripathi, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi

Swift heavy ions based techniques are becoming very important in materials synthesis and modifications at nanoscale. The possibility of creating arrays of perfectly parallel conducting carbon nanowires, in fullerene (C60and C70) thin films has been demonstrated. The passage of swift heavy ion results in an electronic energy loss induced increase in conductivity around ion path. The fullerene thin films, turn into amorphous carbon around the ion track core and get polymerized along the ion track halo. Conducting tracks are studied after irradiation with 55MeV Ti, 120 MeV Au and 200 MeV Ag ion beams in C60 films and with and 100 MeV Ag ion beam in C 70 films.

The unirradiated films have a semiconducting nature of I–V characteristic and it shows an increasingly ohmic behavior with increasing fluence and increased conductivity with increasing electronic energy loss. The typical diameter of the conducting wires is observed to be about 50–200 nm for both C60and C70 films. The ion beam technique provides a simple control over the length, average spacing and angle from the surface by controlling the film thickness, ion beam fluence and the angle of irradiation. The conducting tracks in C60 film also show good field emission properties. Irradiation with 120 Mev Au ions shows a reduction in threshold field to 9 V/μm from a high breakdown voltage of 51 V/μm for the as-deposited films.

Monday, 19 July 2010, 3:00 PM in Physics 152

Search for Gravitational Waves from LIGO-Virgo Science Run and Data Interpretation

Rahul Biswas, UWM

Gravitational-wave astronomy would open a new window to the Universe. A network of Earth-based detectors, including LIGO in the U.S. and Virgo in Italy, are fully operational and have jointly collected data during LIGO's fifth science run (S5) and Virgo's first science run (VSR1).

In this talk, we shall see how Einstein's general theory of relativity predicts the existence of gravitational waves. We shall illustrate the mechanics behind the generation of gravitational waves by binary neutron stars. The rest of my talk will focus on the method used in the search for gravitational waves from binary neutron stars followed by the results of the analysis of S5/VSR1 data including the upper limit on the rate of events as set by this analysis

Friday, 16 July 2010, 3:00 PM in Physics 152

Application of Optical Spectroscopic Techniques for Disease Diagnosis

Anushree Saha, UWM

Molecular specific sensing and monitoring systems with high sensitivity, specificity and spatial resolution have an enormous potential impact on medical diagnostics, therapies, and fundamental biomedical research. Optical spectroscopy, a truly non-invasive tool for remote diagnostics, is capable of providing valuable information on the structure and function of molecules. However, most spectroscopic techniques suffer from drawbacks, which limit their application.

Raman spectroscopy, which requires minimal to no sample preparation, is considered to be the most suitable tool for studying biological molecules, but suffers from two major drawbacks such as fluorescent background and very weak signal level. As a part of my dissertation work, I have developed theoretical and experimental methods to address the above mentioned issues thereby making them more versatile for clinical applications. I have successfully applied these methods for monitoring physical, chemical and biochemical parameters involved in some specific life threatening diseases.

Wednesday, 9 June 2010, 3:30 PM in Physics 135

Growth and Magnetoelectric Properties of Multiferroic Rare-Earth Manganites

Mark Williamsen, UWM

GdMnO3 and TbMnO3 belong to a class of oxide materials which have become interesting in recent years due to indications that an ordered magnetic ground state co-exists with an ordered ferroelectric ground state in the same crystalline phase. A striking aspect or “magnetoelectric property” in some of these systems is that ferroelectric ordering (resulting from ordered dipole moments in the system, and measured by a change in dielectric property) occurs at low temperature, below the Neel temperature at which Mn spins order. This is interesting because, whereas conventional ferroelectricity often shows up at high temperature, together with a change in crystal structure, these systems transition into ferroelectric order at low temperature. Might the appearance of magnetic order drive ferroelectric order? Before jumping to conclusions about the fundamental mechanism of these fascinating phenomena, it is important to search for–and organize–clues from careful measurements on high-quality single crystals.

Tuesday, 14 May 2010, 3:30 PM in Physics 135

Type Ia Supernovae: The Biggest Nuclear-Powered Explosions in the Universe

Robert Fisher, UMASS-Darmouth

In this talk, I will discuss the biggest nuclear-powered explosions in the universe — an amazing stellar explosion known to astronomers as “Type Ia supernovae.” Type Ia supernovae are so incredibly bright that they can outshine the combined stellar light of an entire galaxy, and be visible across enormous distances in the cosmos. Yet, each Type Ia supernova event has very nearly the same intrinsic brightness regardless of where or when in the universe it exploded. Consequently, they provide us with standard candles which have enabled precision cosmological measurements of Hubble's constant and of the acceleration parameter of the universe, and led directly to the existence of a mysterious new type of energy, which has been termed “dark energy.”

I will discuss recent work which has begun to unravel the mystery of these remarkable explosions, and for the first time, provided a self-consistent theoretical explanation of the mechanism underlying their detonation. My talk will include stunning, award-winning visualizations of supercomputer simulations of Type Ia supernova explosions, which are based upon rigorous physics, and have helped to elucidate the complex nature of the explosion.

Friday, 7 May 2010, 3:00 PM in Physics 135

Electroweak stars: Electroweak Matter Destruction as Exotic Stellar Engine

Dejan Stojkovic, SUNY-Buffalo

Stellar evolution from a protostar to neutron star is of one of the best studied subjects in modern astrophysics. Yet, it appears that there is still a lot to learn about the extreme conditions where fundamental particle physics meets strong gravity regime. After all of the thermonuclear fuel is spent, and after the supernova explosion, but before the remaining mass crosses its own Schwarzschild radius, the temperature of the central core of the star might become higher than the electroweak symmetry restoration temperature. The source of energy, which can at least temporarily balance gravity, are baryon number violating instanton processes which are basically unsuppressed at temperatures above the electroweak scale.
We constructed a solution to the Oppenheimer-Volkoff equation which describes such a star. The energy release rate is enormous at the core, but gravitational redshift and the enhanced neutrino interaction cross section at these densities make the energy release rate moderate at the surface of the star. The lifetime of this new quasi-equilibrium can be more than ten million years, which is long enough to represent a new stage in the evolution of a star

Friday, 30 April 2010, 3:00 PM in Physics 135

Differential Multiphoton Microscopy

Jeff Squier, Colorado School of Mines

Visualizing and understanding biological structure and function is inherently a three-dimensional challenge. To date, multiphoton microscopes have established themselves as essential tools in meeting this challenge – they can provide detailed information at high resolution deep within scattering media. The multidimensional aspect of the specimen is typically explored sequentially – a sharp two-dimensional image is created by rastering the excitation beam within a fixed focal plane, followed by a repositioning of the specimen relative to this focal plane, and the process repeated. The tremendous average power available from standard femtosecond lasers suggest a new paradigm for image acquisition – a redistribution of the excitation light such that multiple focal planes can be acquired simultaneously as opposed to sequentially. Significantly, this new model must not come at the cost of sacrificing one of the most beneficial aspects of multiphoton imaging – its ability to image deep into scattering tissue. Thus, a non-imaging modality must be retained – single element detection (for each) modality is optimal.

We have developed a new imaging system that, for the first time, successfully addresses these issues and is denoted as differential multiphoton microscopy. Implementation of this new technique will be described, and results in a range of biological specimens presented.

Friday, 23 April 2010, 3:00 PM in Physics 135

Teaching Robots to See

Yann LeCun, Silver Professor of Computer Science and Neural Science Courant Institute of Mathematical Sciences and the Center for Neural Science of New York University

The visual systems of animals and humans learn to locate and recognize objects, to recognize locations, and to navigate the world autonomously and effortlessly. What “learning algorithm” does the visual cortex use to organize itself? Devising architectures and algorithms that can learn complex visual tasks but just looking at the world, the way animals do, is a major challenge for machine learning and computer vision. The visual cortex uses a multi-stage hierarchy of representations, from pixels, to edges, to motifs, to parts, to objects, to scenes. Current research in so-called “deep learning” is producing new algorithms that learn such multi-stage hierarchies of representations in an unsupervised fashion.

I will first describe the convolutional network model, whose architecture is inspired by the visual cortex. Each stage is composed of a series of filters, followed by a non-linear operation, and a spatial pooling operation that builds invariance to small geometric transformations of the input. The learning algorithm captured dependencies between variables (pixels, features, or labels) by associating an energy to each configuration of variables. The learning procedure shapes the energy landscape so as to give low energy to observed configurations and high energy to every other configurations. Our favorite strategy for shaping the energy surface relies on using sparsity constraints on the features.

An application to category-level object recognition with invariance to pose and illumination will be described. A real-time demo will be shown. Another application to vision-based navigation for off-road mobile robots will be shown. After a phase of off-line unsupervised learning, the system autonomously learns to discriminate obstacles from traversable areas at long range using labels produced with stereo vision for nearby areas. Other applications of energy-based deep learning to time-series prediction, and house price prediction will be briefly presented.

Tuesday, 20 April 2010, 3:30 PM in Physics 135

Competing Ground States in Multiferroic Complex Oxides

Shishir Ray, UWM

Intrinsic magnetoelectrics’ are multiferroic compounds in which long-range ordered magnetism and ferroelectricity occur simultaneously in the same intrinsic crystalline phase. Such ordered ground states not only occur in the same phase: they are coupled. This means that switching the applied electric field (voltage) on a crystal can switch its macroscopic magnetic moment. Similarly, an applied magnetic field can change the macroscopic electric polarization. These highly unusual properties provide the tantalizing possibility of applications in switching, data storage, and the broadly defined area of spintronics.

In an effort to understand the fascinating physics of these systems, I have synthesized and studied an array of multiferroic compounds in single crystal and nano forms, including complex oxides of Manganese and Titanium. My studies have included synthesis of single crystals, Rietveld and Pair Distribution function refinement of crystal structures, measurements of a series of physical properties such as specific heat, ultrasound velocity, magnetic moment and dielectric constant with variable temperature (0.35 Kelvin -1300 Kelvin) and magnetic field (0-9 Tesla), with further measurements performed at the Advanced Photon Source, the Synchrotron Radiation Center, the National High Magnetic Field Laboratory, and Los Alamos National laboratory.

In this talk, I will focus on YbMnO3, a hexagonal compound in the (RE)MnO3 family, known to undergo a transition into a ferroelectric state at 970K, together with antiferromagnetic order of Mn3+ below TN~80K, and Yb3+ order at lower temperature. Based on my observations, we have begun to understand the nature of the transitions between ordered Yb3+ and Mn3+, and uncovered two separate ordering transitions of Yb: Yb3+ (2a) via Yb-Yb, and Yb3+ (4b) via Yb-Mn interactions within the hexagonal YbMnO3 structure. These new results are consistent with recent Neutron Diffraction and Mossbauer studies, and have helped us make headway into understanding the magnetically ordered ground states competing at low temperature in the magnetic (H-T) phase diagram of YbMnO3.

Friday, 16 April 2010, 3:00 PM in Physics 135

High-resolution Electron Microscopy and Spectroscopy of Nanostructures

Velimir Radmilovic, Lawrence Berkley National Lab

This presentation will illustrate the importance of understanding the fundamental features that underlie the behavior of nanoscale phases embedded in a solid matrix and their role in the evolution of microstructure in materials. Because of the scale and nature of such microstructures, high resolution electron microscopy and spectroscopy are essential tools in their characterization.

In the first part of my presentation, I’ll be talking about monodisperse Al3(ScLi) core/shell ordered nanostructures with a Sc and Li-rich core surrounded by a Li-rich shell that can be created via a three-stage heat treatment. Their atomic structure has been studied by a range of microscopy, spectroscopy and synchrotron radiation techniques combined with first-principles calculations of interaction between Sc and Li confined in the ordered phases. Conventional high-resolution phase contrast imaging reveals the fully ordered L12 structure of the shell. Atom-probe tomography reveals that Sc is present in the core while Li is present in both, the core and the shell. Aberration corrected transmission electron microscopy was employed to image Li using exit wave reconstruction. The phase of the exit wave distinguished clearly Al columns from Li columns in the Li rich L12 shell. A detailed analysis of these nanostructures has provided important insights into their atomic structure and composition.

In the second part of my talk, I’ll be showing several examples of application of advanced characterization techniques to study variety of nanostructures for energy related applications, such as: graphene, Pt/Pd core/shell catalysts, ZnO-In2O3 thermoelectric nanowires and amorphous/nanocrystalline composites for MEMS/NEMS applications.

Friday, 9 April 2010, 3:00 PM in Physics 135

Clinical Applications of Low Energy X-rays for Non-Melanoma Skin Cancer Treatment

Yi Rong,University of Wisconsin (Human Oncology) and University of Wisconsin Cancer Center (Radiation Oncology)

Non-melanoma skin cancer was estimated to affect more than 1 million persons in the United States annually. The high dose rate (HDR) Brachytherapy with Ir192 radioisotope is widely used for treatment of this type of cancer. The low energy miniature X-ray source (Xoft Axxent®) provides dose rates comparable to 7Ci Ir192 source, with similar dose distributions for skin cancer treatments. The new surface applicators, with four available sizes including 10mm, 20mm, 35mm, and 50mm in diameter, were cleared by the FDA in March 2009 and first used for patient treatments at our institute in July 2009. So far ten cases for a total of nine patients with basal cell carcinoma or squamous cell carcinoma were treated with the Xoft Axxent® system and low-energy x-ray source (50-kVp), and/or with concurrent Chemotherapy using Efudex Fluorouracil (5-FU). We report a comprehensive quality assurance (QA) procedure for the commissioning of the Electronic Brachytherapy (eBx) system with the surface applicators. Pre-treatment calibration includes dose-rate measurement, air-gap factor, energy verification, beam flatness and symmetry, and treatment planning with patient specific cutout factors. The average nominal dose-rate output at the skin surface for the 35mm applicator is 1.35 Gy/min with ±5% variation for fifteen sources. For the same source, the output variation is within 2%. Treatment regimen includes 45 Gy in 15 fractions of 3 Gy each (3 fractions per week), or 40 Gy in 8 fractions of 5 Gy each (2~3 fractions per week). Physician identified the PTV with a 2~10mm expended margin to the lesion and a prescription depth of 2~6 mm, depending on the diagnosis. Patients were scheduled for follow-up re-evaluations after completion of therapy. The Xoft Axxent eBx with the surface applicator performed well clinically for non-melanoma skin cancer.

Friday, 2 April 2010, 3:00 PM in Physics 135

Using the Universe as a Microscope to See the Small Scale Structure of Spacetime

Lee Smolin, Perimeter Institure (Ontario)

I will review recent observations by the FERMI gamma ray observatory and other astrophysical observations which probe the structure of spacetime at Planck scales. These appear to be close to answering the question of whether the symmetries of special relativity are preserved, broken or deformed by the fundamental structure of quantum spacetime. Different approaches to quantum gravity lead to different expectations or predictions for these questions, making these observations highly relevant for the unfinished task of unifying quantum physics with general relativity. Among the approaches I will describe is deformed (or doubled) special relativity, which modifies the postulates of special relativity to preserve an energy scale as well as a velocity.

Friday, 12 March 2010, 3:00 PM in Physics 135

Imaging with coherent X-rays

Keith A Nugent, ARC Centre of Excellence for Coherent X-ray Science, School of Physics, University of Melbourne.

Coherent diffractive imaging (CDI) is a rapidly emerging area of research that is now beginning to see application to important problems in biology and materials science. In this talk I will review the historical development and current state of the field with a particular emphasis on work using synchrotron sources and free electron lasers. 

Modern X-ray lasers are not perfectly coherent and so I will also discuss the impact of the partial coherence of the light on the ability to reconstruct a reliable high-resolution image. In particular, I will discuss the work of my group in which we have measured the coherence properties and included them into the imaging process.

An important potential application of CDI is to molecular imaging with a free electron laser. I will argue that the impact on the intense X-ray field on the molecule can be treated as a form of partial coherence and discuss how these ideas might ultimately be used to account for the effects of damage from the laser pulse.

Friday, 5 March 2010, 3:00 PM in Physics 135

Quantum Coherence in Molecular Gasses and Solids: Physics and Applications

Alexei Sokolov, Department of Physics, Texas A&M University

I will review the advancements that have resulted in extending the ideas of atomic coherence to molecular physics. We have developed a Raman light source with a bandwidth spanning infrared, visible, and ultraviolet spectral regions, and capable of producing ultra-short (single-cycle) pulses of light which are automatically synchronized with respect to the molecular oscillations. The central feature of our technique is the preparation of an ensemble of molecules in a coherent superposition state — a feature that has earlier been used in electromagnetically induced transparency, ultraslow light propagation, and lasing without inversion. Furthermore, our work has shown that an increased and cleverly manipulated Raman coherence enables improvements in optical microscopy, detection and sensing applications.

Friday, 26 February 2010, 3:00 PM in Physics 135

Coherent X-ray Diffraction Imaging at the Advanced Photon Source

Ross Harder, Advanced Photon Source, Argonne National Laboratory.

The Advanced Photon Source (APS) at Argonne National Laboratory is a third generation synchrotron with sufficient coherent flux to enable the development of novel imaging methods with x-rays. By measuring the coherently scattered photons from samples, and computationally inverting the intensities to an image, we gain two important advantages. We can overcome the limits of resolution imposed by x-ray lenses and increase the space available around the sample to permit in-situ imaging during a variety of experiments.

We have pioneered a unique form of coherent x-ray diffraction imaging at the APS with yet another advantage. By measuring the coherent scattering in the vicinity of Bragg peaks of crystalline samples we gain high sensitivity to distortion of the crystalline lattice due to strain. 

Very recently we have developed instrumentation and methods that allow us to image, in three dimensions, the total lattice deformation field of micro and sub-micro meter size crystals with high spatial resolution. Using this data the entire strain tensor of the sample can be determined in 3D.

This talk will introduce the coherent diffraction imaging done at the APS with a particular focus on the work being done in the Bragg geometry.

Friday, 19 February 2010, 3:00 PM in Physics 135

Surface Structural Functionality of Correlated Electronic Materials

V. B. Nascimento, Dept. of Physics and Astronomy, Louisiana State University

An atomic level knowledge of the structure is a key point in understanding the physical properties of any material. This statement applies to a wide variety of systems, ranging from DNA to complex crystalline materials such as transition metal compounds. As many of transition metal compounds exhibit strong correlation between lattice, charge, spin and orbital, subtle structural distortion often lead to dramatic changes in the physical properties. In situ creation of a crystallographic surface by cleavage offers a unique controllable way to understand the effects of lattice distortions in the material functionality — the intrinsic lack of translational symmetry at the surface will induce structural distortions at the top atomic layers.

As a consequence new phases will be present at the surface with functionality very distinct from bulk. The Low Energy Electron Diffraction (LEED) technique is arguably the most reliable technique to analyze the structure of surfaces. The combination of LEED with other surface sensitive characterization techniques and theoretical modeling results in a powerful joint effort to understand the surface structural functionality.

Friday, 12 February 2010, 3:00 PM in Physics 135

The search for compact binary coalescence in association with short GRBs with the first-generation LIGO and Virgo detectors

Nickolas Fotopoulos, UWM

Gamma-ray bursts (GRBs) are fantastically powerful, faraway explosions. Every day, we detect light from these explosions with the several spacecraft that form the interplanetary network, but their mysteries endure. For a subset of bursts, the short-duration GRBs, the prevailing progenitor theory is that the explosions occur in the final
stages of the inspiral of a neutron star with either another neutron star or a black hole. Such cosmic violence would make itself known also through an outpouring of gravitational waves. Detection of these gravitational waves would not only solve the astronomical puzzle of the origin of short GRBs, but also provide measurements of the
equation of state for matter at super-nuclear densities, measure the dark matter and dark energy content in the Universe, and allow tests of Einstein's general relativity in the strong-field regime. I will describe a search for these gravitational waves using detectors from the Laser Interferometer Gravitational-wave Observatory (LIGO) and
Virgo in their first-generation configurations.

Friday, 22 January 2010, 3:00 PM in Physics 135

Can we observe atoms at work? The TEAM project and its impact on science.

Christian Kisielowski, National Center for Electron Microscopy and Helios SERC, Lawrence Berkeley National Laboratory 

This seminar addresses advances in electron microscopy that were accomplished over the past years by stabilizing microscope columns and by the incorporation of new electron-optical components such as aberration correctors, monochromators and high brightness guns. Many of these developments were pursued within the DoE’s TEAM project that was recently concluded [1]. As a result a new generation of electron microscopes is now available that has reached 50 pm of resolution, which is limited by the Coulomb scattering process itself and not longer by instrument performance [2]. The extraordinary performance of these microscopes has sparked a debate about how to harvest the gained instrumental abilities for the benefit of science. There are many aspects that can be considered in this discussion and one outstanding feature is that the resolution improvement has helped boosting the signal to noise ratio for the detection of single atoms across the Periodic Table of Elements (Z  > 2). Consequently it became possible to image single atoms routinely even if they are light (Z < 10) [3] and to identify the atomic structure of single point defects. Even the three-dimensional location of self-interstitial atoms in crystals can be revealed. New research areas such as the quest for more sustainable energy solutions largely benefit from this development since catalytic materials and polymers can be imaged time resolved at atomic resolution with single atom sensitivity. In essence we start seeing atoms at work within the ultimate limit that is given by radiation damage.