Colloquia Archive: 2007

Friday, 26 January 2007, 3:00pm

Guenther Paltauf, Dept. of Physics, Karl-Franzens – Universitaet Graz, Austria
Photoacoustic Tomography (PAT) with Optical Ultrasound Detection
Location: Physics 135

Photoacoustic tomography is an emerging biomedical imaging technique that combines optical contrast with ultrasound resolution. It is based on the generation of sound in a semitransparent medium by irradiation with pulsed electromagnetic radiation. Thermal expansion generates sound waves that are measured with ultrasound detectors outside the imaged object. A three-dimensional image of the distribution of absorbed electromagnetic energy is obtained by applying reconstruction algorithms to the recorded temporal pressure signals. In this talk, various methods are presented to generate three-dimensional PAT images from signals recorded with optical ultrasound detectors. Optical detection has various advantages compared to conventional piezoelectric detection. First of all, the size of optical detectors can be made very small, approaching the wavelength of light. This is important for achieving high imaging resolution. Furthermore, optical detectors can be made totally transparent, which facilitates the pulsed optical excitation of ultrasound in a sample. Finally, various shapes of detectors can be easily realized with optical methods, in particular small point like or extended line receivers. Examples of detectors based on optical reflectance at an interface and on optical interference are presented. Detector properties, such as temporal resolution and sensitivity are shown and methods for image reconstruction specific to the detector size and shape are discussed.

Tuesday, 30 January 2007, 3:00pm

Pratap Raychaudhuri, Dept. of Condensed Matter Physics & Materials Science, Tata Institute of Fundamental Research (Colaba, Mumbai, INDIA)
Probing Superconductors using Point Contact Andreev Reflection
Location: Physics 135

Point Contact Spectroscopy relies on one of the oldest circuit elements, i.e. an electrical contact between two metals, to obtain information on the electrons close to Fermi level. In this kind of experiments an electron is “accelerated” by applying a voltage across a ballistic contact between two metals. At energies characteristic to the elementary excitation in the metal, the electron loses its energy leaving signatures in the conductance characteristics of the contact. When a ballistic contact is established between a normal metal (N) and a superconductor (S), the transport is dominated by the process called Andreev reflection, where an electron incident on the (N/S) interface from the normal metal gets reflected as a hole and a Cooper pair propagates in the superconductor. Point Contact Andreev Reflection (PCAR) provides a convenient tool to obtain a variety of information such as the superconducting energy gap and the pairing symmetry of a superconductor. In this talk I will discuss two of our recent investigations using PCAR. In the first part I will discuss the magnetic field and temperature dependence of the unusual gap anisotropy in the quaternary borocarbide superconductor YNi2B2C and show that our results provide strong indication of multiband superconductivity in this material. In the second part, I will discuss our results on the size dependence of the superconducting energy gap in nanocrystalline Nb. Finally, I will highlight some of the pitfalls of this technique and discuss how one can avoid them.

Friday, 2 February 2007, 3:00pm

Michael Stoneman, University of Wisconsin-Milwaukee
Protein Influence on the Plasma Membrane Dielectric Properties: towards tag-free detection of proteins and their interactions
Location: Physics 135

Dielectric spectroscopy has been used as a technique to probe biological systems since the early twentieth century. As early as 1925, at a time when microscopy techniques were not developed sufficiently to study the cell membrane, Henry Fricke determined its thickness using dielectric measurements. With the advent of fast impedance analyzers during the 1980's, highly accurate large scale studies of biological systems have become more feasible. Recent studies using dielectric spectroscopy have reported on the structures of numerous types of cell suspensions and tissues, and also time-dependent processes involving them, such as cell sedimentation, cell aggregation, cell division, and tissue viability after excision. The underlying principles of dielectric spectroscopy are rather simple. The dielectric properties of a material (permittivity and conductivity) are measured and analyzed as a function of frequency. Protein localization, activity, and interactions in living cells are increasingly important areas of research, for both industrial applications and basic fundamental biological knowledge. A popular method of investigation in this area is fluorescence imaging, which is a widespread and well-developed biological tool of investigation. Fluorescence imaging attaches a fluorescent marker to a protein of interest, and monitors the location and activity of the protein through the marker. Such a method is critical to the advancement of knowledge in various subfields of biology; however, ideally one wishes to be able to detect proteins and protein interactions without relying on tagging, which might perturb the system under study. It has been proposed recently that detection of dielectric properties of the cell may eventually lead to noninvasive methods for detection of receptor proteins on the plasma membrane and for monitoring their activity and interactions in vivo, without recourse to tagging. In this talk I will (a) present our recent investigations that combined the techniques of Dielectric Spectroscopy and Fluorescence Imaging to determine the contributions of the main molecular components of the plasma membrane to the measured plasma membrane permittivity, and (b) discuss future studies that may lead to non-invasive detection in vivo of protein and protein interactions on the cellular membranes by means of dielectric spectroscopy.

Friday, 9 February 2007, 3:00pm

Vlad Yakovlev, Assoc. Professor, University of Wisconsin-Milwaukee
“Shall We Dance?”
Location: Physics 135

Ever since the discovery of the microscope, the imaging of a single molecule was always presented as one of the greatest challenges for scientists. Fifty years ago, Schrödinger wrote that he did not believe that it would ever be conceivable. Today, of course, due to numerous technical improvements, it is possible to watch the motion (“dance”) of individual single molecules by monitoring, in a real-time, their position with nanometer precision and measuring forces generated by molecules with piconewton accuracy. This allows Physics to step into a world of Biology by introducing a quantitative analytical approach to long-standing problems. In my talk I will introduce some basic biology necessary for understanding the physics of biological molecular motors, and show how single-molecule spectroscopy can minimize some of the mysteries in this field, while leading to a better understanding of how the motion of those molecules can be initiated by variation of physical, rather than biochemical, parameters. I will also make a connection to my previous work on coherent control and will evaluate on what does it take to control biological molecules and, hence, biological processes with light waves.

Monday, 19 February 2007, 3:00pm

Yuhong Wang, Postdoctoral Scholar, Univ. of Pennsylvania, Dept. of Chemistry & the Pennsylvania Muscle Institute
Single Molecule FRET Study of Structural Dynamics During Protein Synthesis
Location: Physics 135

During protein synthesis, EF-G catalyzes translocation of mRNA and tRNAs within the ribosome during protein synthesis. Detection of structural states in the reaction sequence that are not highly populated can be facilitated by studying the process one molecule at a time. I will present single molecule studies of translocation showing that, for ribosomes engaged in poly(Phe) synthesis, fluorescence resonance energy transfer (FRET) between the G' domain of EF-G and the N-terminal domain of ribosomal protein L11 occurs within two rapidly interconverting states, having FRET efficiencies of 0.3 and 0.6. The antibiotic fusidic acid increases population of the 0.6 state, indicating that it traps the ribosome EF-G complex in a pre-existing conformation formed during translation. Only the 0.3 state is observed when poly(Phe) synthesis is prevented by omission of EF-Tu, or in studies on vacant ribosomes. These results suggest that the 0.6 state results from the conformational lability of unlocked ribosomes formed during translocation. An idling state, possibly pertinent to regulation of protein synthesis, is detected in some ribosomes in the poly(Phe) system.

Thursday, 1 March 2007, 3:00pm

Weidong Yang, Dept. of Molecular & Cellular Medicine (The Texas A&M Univ. System Health Science Center)
Single Molecule Studies of Translocation Through Nuclear Pore Complexes
Location: Physics 133

While many components and reaction steps necessary for bidirectional transport across the nuclear envelope (NE) have been characterized, the mechanism and control of cargo migration through nuclear pore complexes (NPCs) remains poorly understood. Here, we demonstrated that single cargo molecules could be visualized interacting with nuclear pore complexes in permeabilized cells by single molecule narrow-field epifluorescence microscopy. At low importin \beta concentrations, about half of the signal-dependent cargos that interacted with an NPC were translocated across the NE, indicating a nuclear import efficiency of ~50%. The NPC interaction times for cargos that actually transported through the NPC and those that underwent abortive transport were both ~8.3 ms under these conditions. At high importin \beta concentrations, the import efficiency increased to ~80% and the transit speed increased ~7-fold. The transit speed and import efficiency of 10 kDa dextran, a signal-independent cargo, was also increased by high importin ? concentrations. These results demonstrate that maximum nucleocytoplasmic transport velocities can be modulated by at least ~10-fold by the importin ? concentration. Thus, we postulate that cargo trafficking rates depend on pore occupancy, a model explicitly recognizing that the number, identity, and distribution of molecules within the pore affects cargo translocation through the pore. Factors that could be affected by changes in pore occupancy include: i) the accessible volume for the transiting cargo; ii) the concentration and distribution of RanGTP in the NPC; and iii) the structure and physical properties of the permeability barrier.

Friday, 2 March 2007, 3:00pm

Xavier Siemens, California Institute of Technology
Gravitational waves: A new observational window
Location: Physics 135

The Laser Interferometer Gravitational-wave Observatory (LIGO) instruments are the most sensitive gravitational wave detectors in the world. In this talk I discuss basic properties of gravitational waves, argue why we should look for them, and show how we might detect them using LIGO. I describe the gravitational wave searches currently underway, and concentrate on three analysis efforts. The first is the calibration, which is the procedure used to turn the digital read-out of the interferometers into the gravitational wave strain incident on the interferometer. The second is the search for gravitational waves from cosmic (super)strings. Finally, I will discuss the search for gravitational waves from rotating neutron stars.

Monday, 5 March 2007, 3:00pm

Badri Krishnan, Max-Planck-Institut fur Gravitationsphysik, Golm
Gravitational Wave Astrophysics of Neutron Stars and Black Holes
Location: Physics 133

The detection of gravitational radiation from astrophysical sources will allow us to observe matter in extreme conditions and to probe properties of spacetime in the presence of dynamical and strong gravitational fields. With the current generation of ground based detectors such as LIGO operating at unprecedented sensitivity, and the planning for future detectors such as LISA well underway, prospects for gravitational wave astronomy have never been more promising. However, even with these detectors operating as designed, detecting and interpreting gravitational radiation remains a challenging task. It requires input not only from gravitational wave data analysis, but also from astrophysics, and numerical and mathematical relativity. In this talk, I discuss this in the context of two promising sources of gravitational waves: rapidly rotating neutron stars, and coalescing binary black hole systems.

Friday, 9 March 2007, 4:00pm

David McMillen, Dept. of Chemical & Physical Sciences and Institute for Optical Sciences, Univ. of Toronto at Mississauga
Ways Cells Differ from Breakers
Location: Physics 135

Genes are not isolated entities; rather, they participate in complex coupled reactions, producing proteins that act to regulate the expression of other genes. The complex set of processes can in principle be reduced to a set of highly coupled chemical reactions, but in practice linking biology and chemistry is a significant challenge, and one that will require the close collaboration of experimental and theoretical approaches. A detailed knowledge of biochemical dynamics would enable us to complete the work of sequencing projects like the Human Genome Project, by augmenting the list of nodes in the genetic network with the dynamics of the network behavior. If cells were simple chemical reaction vessels, then identifying the biochemical reactants and measuring rate constants and concentrations would allow us to write down the chemical kinetics of each reaction involved in gene expression and regulation, and the task would be difficult only in that there would be a great many individual measurements to be made. In fact, however, cells differ significantly from beakers, and this adds complexity to the problem of understanding their behavior. I will present recent experimental and computational work in our lab, in the context of describing these complicating factors. Topics will include: quantification of protein numbers and the “dark protein” problem; cell size and cell-cycle effects; isolation of extrinsic noise sources in E. coli; and a simplification method for reducing the complexity of biochemical models.

Monday, 12 March 2007, 3:00pm

Rhonda Dzakpasu, Dept. of Physics, University of Michigan
Causal Entropy as a Measure of Temporal Relationships and Direction of Information Transfer in Neural Systems
Location: Physics 135

There are billions of neurons in the brain, each of which participates in the execution of various functions. How does the brain organize the operations of these fundamental units? What relationships exist, on a temporal scale, between neurons? Is there an ordering between the temporal patterns of neurons? In order to begin to address these questions, we have developed a novel analytical tool that measures temporal interdependencies between coupled neurons. The technique involves the real time monitoring of inter-event intervals between the coupled neurons. We demonstrate the feasibility of the measure on a mathematical model co nsisting of two, coupled non-identical Hindmarsh-Rose models of thalamo-cortical neurons. We show that the measure may be better than more conventional methods at detecting changes in asymmetrical temporal patterns. Finally, we demonstrate how the technique can be modified to study networks of coupled neurons and discuss the application of the measure in the analysis of experimental data.

Tuesday, 13 March 2007, 3:00pm

Duncan Brown, California Institute of Technology, LIGO Laboratory and Theoretical Astrophysics
Listening For Black Hole Spacetimes with LIGO
Location: Physics 133

Gravitational waves from the inspiral and coalescence of binary compact objects, such as a pair of neutron stars or black holes orbiting around each other, are one of the most promising sources for the Laser Interferometer Gravitational-wave observatory (LIGO) and its international partners. With the first generation detectors now operating at their design sensitivity, we are searching for gravitational waves further into the universe than ever before. Furthermore, the recent developments in numerical relativity now allow us to simulate the dynamics of binary black hole inspirals for more than 15 orbits before the black holes merge. The detection of gravitational waves will open a new window on the universe and give us our first measurements of strong field gravity. I will give an overview of the methods used to search gravitational waves from compact binaries and discuss how numerical simulations are being used to improve both detection of inspirals and extraction of the information contained in the gravitational waves. I will also mention some of the tests of General Relativity that might be accessible to the advanced generation of LIGO detectors.

Thursday, 15 March 2007, 3:00pm

Benjamin Owen, Pennsylvania State University
Why LIGO Results Are Already Interesting
Location: Physics 133

Recent theoretical developments indicate that rotating neutron stars might be stronger sources of periodic gravitational waves than previously thought. LIGO (the Laser Interferometer Gravitational-wave Observatory) has a chance of detecting such a signal right now rather than next decade. A detection could provide unique information on the composition and structure of a star. Even without a detection, LIGO can place upper limits on gravitational waves from several types of neutron stars which beat the indirect upper limits derived from photon astronomy. I survey the LIGO searches for periodic signals and describe how we can turn them into gravitational wave astronomy, right now and in the years to come.

Monday, 26 March 2007, 3:00pm

Vuk Mandic, California Institute of Technology
Searching for Stochastic Gravitational-wave Background with LIGO: Results and Implications
Location: Physics 135

The Laser Interferometer Gravitational-wave Observatory (LIGO) has built three multi-km scale interferometers, designed to search for gravitational waves (GW). One of the targets for these searches is the stochastic GW background, whose existence is expected both due to cosmological and due to astrophysical sources. We discuss the status of LIGO, the most recent results of the search for stochastic GW radiation with LIGO interferometers, and the implications of these results for some of the theoretical models of stochastic GW background.

Friday, 30 March 2007, 3:00pm

Dr. Mark A. Anastasio, Associate Professor of Biomedical Engineering & Electrical and Computer Engineering, Illinois Institute of Technology
Recent Advancements in Tomographic Imaging with Coherent Wavefields
Location: Physics 135

Phase-contrast tomographic imaging methods that employ coherent optical or X-ray wavefields aim to reconstruct the complex valued refractive index distribution of an object. In this talk, we describe novel tomographic reconstruction theories and data acquisition strategies for several variants of phase-contrast tomography. For the case of propagation-based X-ray phase-contrast tomography, statistically principled reconstruction strategies are proposed that effective cancel singularities in the reconstruction formulas. We also describe a novel reconstruction method for analyzer-based phase-contrast X-ray imaging, which can measure the ultra-small-angle-scattering as well as refraction properties of an object. For use with optical wavefields, holographic tomographic reconstruction methods are developed by exploitation of symmetries that are inherent in the wave propagation-based imaging models.

Wednesday, 4 April 2007, 4:30pm

Dr. Kyle Hollman, Biomedical Ultrasonics Lab, University of Michigan
Measuring and Modeling Elasticity Distribution in the Intraocular Lens
Location: Physics 137

Current theory holds that presbyopia (the loss of accommodation with age leading to the need for bifocals or reading glasses) is caused by increased stiffening in the lens as we age. While recent measurements of lens stiffness do show an age related increase, they also indicate that stiffness and age-related change in stiffness are not homogeneous. To understand these effects, this study uses a novel ultrasound method to measure the elasticity distribution in a lens and then mathematically models accommodation. For the measurement technique, acoustic radiation force from ultrasound is used to push on a laser-produced bubble in in-vitro lenses. The position of the bubble is tracked with low-amplitude, high-frequency ultrasound. Knowing force and displacement, an elastic modulus can be calculated at every position a bubble is placed, mapping elasticity in a lens. Values from these measurements and other studies have been incorporated into a finite element model. In a model effects from changes in elasticity distribution can be separated from the average increase in stiffness. These measurements and modeling give us a better understanding of lens biomechanics which could lead to a laser-microsurgery correction for presbyopia.

Friday, 13 April 2007, 3:00pm

Dr. Daniel Agterberg, Assoc. Professor, University of Wisconsin-Milwaukee
Using Magnetic Fields to Manipulate Superconductivity
Location: Physics 135

Qualitatively, superconductivity arises when fermions pair to create bosonic Cooper pairs that Bose condense. For this to occur, superconductors require time-reversal or parity symmetries. In particular, to create a Cooper pair from two fermions, the relevant fermion quantum states are related by either of these symmetries. Removing one of these symmetries leads to superconductivity with dramatically new properties. In this talk the controlled removal of time reversal symmetry (through the application of magnetic fields) will be explored in superconducting materials with and without parity symmetry. Magnetic fields play two distinct roles in superconductors. The first is a change in the fermion quantum states that participate in the pairs. The second is the introduction of vortices in the superconducting state. This talk will present recent work that unifies these two different roles with some surprising consequences. A pedagogical background describing magnetic fields and superconductivity will be given. The relevance of this problem to high density quark matter and to Bose condensation of cold atoms will also be presented.

Tuesday, 17 April 2007, 3:00pm

Dr. S. H. (Irene) Cheung, Fundamental Science Directorate, Pacific Northwest National Laboratory (Richland, WA)
Fundamental studies of N incorporation and electronic structure in N-doped TiO2 grown by Plasma Assisted Molecular Beam Epitaxy
Location: Physics 133

N-doped TiO2 is of potential interest for bandgap reduction, enhanced visible solar light absorption, and H2O splitting to produce H2 as a cheap and plentiful energy source. Critically important questions include N speciation, the mechanism by which N is incorporated into the lattice, the maximum achievable dopant concentration, and the effect of dopant concentration on photocatalytic activity. In order to address these issues, we have undertaken a detailed study of TiO2-xNx rutile films grown by plasma assisted molecular beam epitaxy. A mixed beam of atomic N and O radicals was prepared in an electron cyclotron resonance plasma source and Ti was supplied from a high-temperature effusion cell or an electron beam evaporator, depending on the required flux. A very high degree of structural quality is generally observed for films grown under optimized anion-rich conditions. Our results reveal that epitaxial growth of TiO2-xNx rutile prepared under anion-rich conditions is accompanied by Ti indiffusion, leading to interstitial Ti (Tii), which is a shallow donor in rutile. Our data strongly suggest that Tii donor electrons compensate holes associated with substitutional N2-. N2- is thus converted to N3-, leading to highly insulating or n-type, but not p-type, material. Indeed, electron paramagnetic resonance (EPR) reveals the absence of Ti(III) and N3-, both of which are EPR active. This result lends significant support for the compensation mechanism Ti(III) + N2- ' Ti(IV) + N3-. Core-level x-ray photoelectron spectroscopy reveals hybridization of N and Ti. Ti – N hybridized states fall in the gap just above the VBM, and extend the optical absorption well into the visible. However, it is not yet clear if this lower absorption threshold constitutes a bona vide band gap reduction. In this talk, we present a summary of our work on these fascinating and potentially important materials.

Thursday, 19 April 2007, 3:00pm

Dr. habil. Marius Schmidt, Dept. of Physics , Technische University – Munchen, Germany
Time, Energy and Space: Variables to Investigate Bio-Molecules
Location: Physics 133

Mössbauer spectroscopy and temperature dependent crystallographic investigations were used to compare the dynamics of the -sheet heme protein Nitrophorin IV with that of the pure -helical protein Myoglobin. Nitrophorin IV becomes functional only at higher temperatures. However, it is more flexible in the physiologic temperature regime and performs larger amplitude motions. These diffusive motions are at the base of conformational changes needed by the protein to perform its function. Conformational changes can be particularly well studied using photoactive proteins since they can be conveniently triggered by light. Of special interest are stable photo-optical switches, which can be actively switched back and forth. The -subunit of the bili-protein Phycoerythrocyanin (PEC) constitutes one of these switches. Here, the bile-chromophore phycoviolobilin undergoes a reversible Z- to E- isomerization. -PEC is a role model for similar reactions in the well-known phytochromes which are the most important photo-receptors in plant and other organisms. The Z- to E-transition is characterized for the first time using conventional static crystallographic methods. However, an authentic, detailed description of the reaction is only possible when time-resolved methods are employed. Time-resolved macromolecular crystallography unifies kinetics with structure determination. Chemical reactions in biological macromolecules can be observed in real-time and on the atomic length scale. For an introduction, photo-flash experiments on L29WMyoglobin are presented which are followed by investigations on the photocycle on Photoactive Yellow Protein. One of the last, biggest challenges was to extract the structures of the transient states and the chemical-kinetic mechanism connecting these states from the time-resolved X-ray data. This problem was approached with great success by a component analysis, the singular value decomposition. Now, structures and kinetics of a chemical reaction can be determined simultaneously and in a global way from the time-course of difference maps. Finally, a short outlook is given covering new approaches to push the limits of time-resolution to the femto-second regime and the crystal size down to the single molecule level.

Friday, 20 April 2007, 3:00pm

Dr. Michael A. Fiddy, Director, Center for Optoelectronics and Optical Communications (Univ. of North Carolina – Charlotte)
Cloaking and Slow Light Structures
Location: Physics 135

The challenges in solving the inverse scattering problem in imaging applications will be outlined. The compliment to this would be an inversion algorithm that permits the so-called structure synthesis problem to be addressed. Two examples of designing scattering structures with prescribed characteristics will be discussed. The first is a 1D photonic crystal that can slow the group velocity of a propagating pulse. The required structure is proving difficult to make at optical frequencies and the inversion-design algorithm remains elusive. The second example is less ambitious and attempts only to control the spatial and not temporal properties of the scattered field. For this a nonlinear signal processing method has been developed and is applied to the design of a strongly scattering object with a defined angular scattering response. Here k-space design methods are combined with cepstral filtering to estimate a permittivity distribution that scatters with the specified response. Simulations are presented which illustrate the usefulness of this method to design cloaking (“invisible”) objects, or structures that might emulate scattering from quite different objects (i.e. shape-shifting).

Friday, 4 May 2007, 3:00pm

Dr. Nikolai Smolin, Daggett Laboratory, Dept. of Medicinal Chemistry; Univ. of WA-Seattle
Role of Hydration Water in Biological Functions: Percolation transition of hydration water in biosystems
Location: Physics 135

Biological functions are possible only in the presence of water, which is essential for conformational stability and dynamics of biomolecules. The physical origin of the crucial role of water in biological functions remains unclear. Functions of biomolecules, as well as properties of water at the surface (hydration water) of biomolecules, drastically depend on temperature, pressure, hydration level, and presence of cosolvents. Understanding of the possible states of surface water at various thermodynamic conditions and their relation to the properties of biomolecules should clarify which properties of water are crucial for biological functions. It was found that proteins rapidly recover their biological activity when the hydration level is increased to the threshold, at which the ensemble of finite hydrogen bonded water clusters is transformed into an infinite hydrogen bonded water network. In this talk I will present the first computer simulations study of the clustering of the water on the planar hydrophilic surface, hydrophilic sphere, single protein molecule and in model protein powder. The percolation transition of the surface (hydration) water was studied through analysis of the cluster size distribution, mean cluster size, size and fractal dimensionality of the largest water cluster. The developed method allows one to locate the percolation threshold of the hydration water in various systems by computational analysis. This opens up new possibilities to use computer simulations to clarify the role of hydration water in dynamics and functionality of biological molecules. I will also discuss behavior of the water surrounding the antifreeze proteins using molecular dynamics method.

Wednesday, 13 June 2007, 3:00pm

Harry Trevor Johnson-Steigelman, UW-Milwaukee Dept. of Physics and Lab for Surface Studies
Surfactant Mediated Growth of Cobalt on Magnesium Oxide(111)
Location: Physics 135

Cobalt-based materials are excellent candidates for tunneling magnetic junctions (TMJ). These structures involve layers of magnetic materials separated by thin, non-magnetic layers of an insulating material. The structure and behavior of the interfaces of these layers have important consequences for the overall function of the TMJ. Monolayer films of Co have been deposited using an electrostatic electron-beam evaporator on single-crystal MgO(111)- and MgO(111)-(1×1)substrates held at room temperature (RT), with subsequent annealing of temperatures 400°C to 800°C. These films have been characterized using low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Upon RT deposition of Co, LEED and XPS signals from the substrate diminish, but return upon annealing. These behaviors suggest 3D islanding, which is corroborated by AFM. The behavior of the Mg 2p and O 1s features may provide insight to previously reported reconstructions of the MgO(111)-(1×1) and MgO(111)- surfaces. The growth mode of a thin film deposited on a surface is determined by the surface energies of the substrate and film, as well as the interfacial energy between the two. A common way to encourage layer-by-layer growth is to use a surfactant. A surfactant lowers the overall surface energy of the system and floats to the growth surface and promoting smooth film growth. Ag has been used successfully as a surfactant for Co in other systems. Ag was examined as a potential surfactant to aid in the growth of smooth Co films. Ag was deposited onto MgO(111)- substrates and investigated using XPS, LEED, and AFM. It was found that Ag formed islands upon annealing. Despite the fact that Ag formed islands, it was found that the presence of Ag did have a surfactant effect upon the thin-film growth of Co on Ag/MgO(111)- substrates with 1-2 ML of Ag. Co islands were still present, but the surface was much smoother than without the Ag surfactant. XPS peak intensity changes and AFM suggest strongly that Ag floated to the top of these samples at temperatures above 400°C. The Co growth mode appears to be Volmer-Weber island growth, as seen in AFM.

Thursday, 14 June 2007, 3:00pm

Hirak Kumar Patra, Dept. of Biochemistry, Univ. of Calcutta — Kolkata, India
Interactions of Synthetic Nanoparticles with Cells
Location: Physics 135

Despite the huge potential benefits of nanomaterials in biomedical and industrial applications, very little information is available about potential short and long-term interaction of such nanomaterials on live cells. We report Gold Nanoparticle (GNP) induced death response in human carcinoma lung cell line A549. At high concentration, GNP accumulates at a region adjacent to the cell nucleus, the clustering being polarized in a small niche around the nucleus. The origin of this optical feature is unclear, but it could be a surface enhanced Raman effect due to clustering of the gold colloid. Region specific cluster formation of GNP's indicates that the gold nanosurface may have a tendency to accumulate in certain cancer cell types, in specific cellular niche and induce a cytotoxic effect.

Thursday, 12 July 2007, 3:00pm

Professor Morten Ring Eskildsen, University of Notre Dame
Vortex Studies in Superconducting CeCoIn5
Location: Physics 135

Since the discovery of superconductivity in CeCoIn5, a plethora of interesting phenomena has been observed in this material. Among these are one of the highest critical temperatures (Tc = 2.3 K) in any heavy fermion superconductor, d-wave pairing symmetry, a paramagnetically limited upper critical field which is first-order at low temperatures, and field- and pressure-induced quantum-critical points and non-Fermi liquid behavior. Finally, several bulk measurements indicate a phase transition to a non-uniform (Fulde-Ferrell-Larkin-Ovchinnikov) superconducting state just below Hc2 at low temperatures. Here we report on small-angle neutron scattering studies of the flux-line lattice (FLL) in CeCoIn5 with fields up to Hc2 = 5 T applied parallel to the c-axis and temperatures from 60 mK to 1.3 K. The FLL undergoes a series of symmetry and reorientation transitions from hexagonal to rhombic to square and back, as the applied magnetic field is increased from zero to Hc2. While the low-field transitions can be understood as driven either by non-local effects coupled to an anisotropic Fermi surface or the d-wave pairing, the high-field behavior is presently unexplained. The magnetic field distribution around the vortices was studied by measuring the FLL absolute scattered intensity which allow a determination of the vortex form factor. The form factor shows a striking departure from the usual exponential decrease with increasing field. Rather, the form factor remains constant in fields up to 2 T, above which it increases. At Hc2 = 5 T the form factor drops abruptly to zero, probably reflecting the first order nature of the upper critical field in this material. While not understood in detail, these results indicate a strong field dependence of the penetration depth and/or coherence length, possibly connected with paramagnetic effects. Comparison will be made with measurements on TmNi2B2C in the paramagnetic state above TN.

Monday, 23 July 2007, 3:00pm

Dr. Steve Asztalos, Physics & Advanced Technologies Directorate Lawrence Livermore National Lab (Livermore, CA)
Chasing Dark Matter Axions to the Quantum Limit and Beyond
Location: Physics 135

The Standard Model predicts that the combined Charge-Parity symmetry in QCD should be violated unless the parameter θ stemming from the non-trivial vacuum is exceedingly small ( < 10-10 ). By contrast, experimental evidence suggests that the strong interactions conserve this symmetry. The most elegant solution to this so-called Strong-CP problem was proposed by R. Peccei and H. Quinn and involves the spontaneous breaking of a new U(1)$_{PQ}$ global symmetry: the axion arises as the associated pseudo-Goldstone Boson. I will review the experiments and astrophysical arguments that lead to an upper bound of 1-10 meV for the mass. Axions with ΜeV mass have not been ruled out and would have sufficient relic density to be a very plausible candidate for cold dark matter. I will summarize current experimental efforts to discover the axion in this mass region, with particular focus on results from and future plans for the Axion Dark Matter eXperiement located at Lawrence Livermore National Laboratory.

Friday, 14 September 2007, 3:00pm

Professor Tom Weiler, Physics Dept., Vanderbilt University
The Most Energetic Photons in the Universe, and How They Are Made
Location: Physics 135

The Nobel prize was given to Einstein in 1921, “. . . especially for his discovery of the law of the photoelectric effect as due to the quantum nature of light.” The quantum is called the photon. In the past two decades, many astrophysical sources of photons having energies ten trillion times that of light have been observed by air-Cerenkov telescopes and other techniques. These astrophysical sources are located both within our Galaxy, and beyond. I will overview this field, and discuss the three main mechanisms for TeV-photon generation that have been suggested. Then I will relate the three mechanisms to source types. The field of TeV gamma-ray physics is rapidly evolving, so my inferences will be dynamic rather than assertive.

Friday, 21 September 2007, 3:00pm

Dr. John Freeland, Advanced Photon Source, Argonne National Lab
Fun and Frustration with Physics at the Interface Between Complex Materials
Location: Physics 135

One of the current problems of intense interest to the condensed matter physics community is the behavior of systems with strongly interacting electrons. These systems contain a variety of competing strong interactions which create a subtle balance to define the lowest energy state (e.g. metal, insulator, superconductor,…). Now, through the use of state-of-the-art sample fabrication, one can create high quality interfaces between dissimilar strongly correlated electron systems. The broken symmetry at the interface though can then drastically upset the subtle balance of these competing energies and lead to significant deviations from the bulk properties. Here, I will review some of our recent work to illustrate how the interface behavior is quite different from the bulk constituents and provides the opportunity to create and study new states created at the boundary [1,2].

Friday, 5 October 2007, 3:00pm

Professor Scott Hughes, M.I.T.
Binary Black Hole Astronomy with LISA
Location: Physics 135

The coalescence of two massive black holes produces gravitational waves (GWs) that will be measurable by the space-based GW observatory LISA. Such binaries are formed by the merger of the holes' host galaxies; hierarchical structure formation predicts many tens of events per year, particularly at redshifts z > 3. Measuring the GWs from these events could thus allow us to track the growth of black holes and -indirectly – trace the assembly of galaxies. Most excitingly, these measurements can provide precision data on the black holes' masses and spins and on the luminosity distance to the source. These parameters can often be measured with 1% accuracy or better. They also localize the source on the sky, though not so precisely: Low redshift merging black holes can be pinned down to within several to several tens of arcminutes; high redshift mergers might only be localized to a degree or so. In this talk, we will summarize the ability of LISA to characterize binary black hole mergers from their measured GWs, showing how the different parameters color the waveform, and how LISA uses those colorings in its measurement. We will also discuss how these measurements can be applied to study the cosmological growth of black holes and (possibly) make precision measurements of the universe's large scale geometry.

Friday, 12 October 2007, 3:00pm

Dr. Robin Santra, Argonne National Lab
Strong-field Control of x-ray Absorption
Location: Physics 135

Strong optical laser fields modify the way x rays interact with matter. This allows us to use x rays to gain deeper insight into strong-field processes. Alternatively, 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 the impact of spin-orbit interaction on the alignment of atomic ions produced in a strong laser field; electromagnetically induced transparency in the x-ray regime; and laser-induced alignment of molecules.

Friday, 19 October 2007, 3:00pm

Dr. Tom Paul, Dept. of Physics, Northeastern University
The Highest Energy Cosmic Rays : Results from the Pierre Auger Observatory
Location: Physics 135

Cosmic rays with energies as high as about 1020 eV have been observed, but determining the origins and composition of particles near this extreme end of the energy spectrum is difficult because of their rarity. The Pierre Auger Observatory has been designed specifically to study such cosmic rays using both a huge array of particle detectors on the ground, covering an area of 3000 square kilometers, as well as a collection of telescopes which observe the fluorescence light produced by particle showers in the atmosphere. The observatory will be completed near the end of this year, but data analyses are already well underway. This presentation will provide a description of the experiment and a report on current measurements of the energy spectrum, hints at chemical composition, and searches for anisotropy in the cosmic ray arrival directions.

Friday, 26 October 2007, 3:00pm

Professor Antonio Delgado, CERN and Physics Dept., Notre Dame University
The Physics Behind the LHC
Location: Physics 135

What do physicists want to discover with experiments at the LHC? What is the Higgs boson? What are the new phenomena that could be observed at the LHC? I will try to answer these questions using language accessible also to non-experts.

Friday, 2 November 2007, 3:00pm

Professor Francis Halzen, Dept. of Physics, UW-Madison
High-Energy Neutrino Astronomy: Towards a Kilometer-Scale Neutrino Observatory
Location: Physics 135

Kilometer-scale neutrino detectors such as IceCube are discovery instruments covering nuclear and particle physics, cosmology and astronomy. Examples of their multidisciplinary missions include the search for the particle nature of dark matter and for additional small dimensions of space. In the end, their conceptual design is very much anchored to the observational fact that Nature produces protons and photons with energies in excess of 1020 and 1013 electronvolts, respectively. The cosmic ray connection sets the scale of cosmic neutrino fluxes. The problem has been to develop a robust and affordable technology to build the kilometer-scale neutrino detectors required to do the science. The AMANDA telescope, using clear deep Antarctic ice as a Cherenkov detector of muons and showers initiated by neutrinos of all 3 flavors, has met this challenge. We review the results obtained with more than 5000 well-reconstructed neutrinos in the 50 GeV ~ 500 TeV energy range collected during its first 4 years of operation. More importantly, we will show that AMANDA represents a proof of concept for the ultimate kilometer-scale neutrino observatory, IceCube, now more than one quarter complete and taking data.

Friday, 9 November 2007 , 3:00pm

Dr. Mark G. Jackson, Fermilab
The State of String Theory
Location: Physics 135

I will present the current situation in Superstring Theory, especially as regards to upcoming experimental prospects. First reviewing the basics of string theory and its modern incarnation M- Theory, I will discuss some 'applications' like black holes, AdS/CFT and brane inflation. I will then address the exciting prospect of upcoming experimental verification along several routes: the LHC, cosmic superstrings, “Transplankian” and CMB physics, heavy-ion collisions, astronomy-based experiments, and novel quantum gravity effects.

Friday, 16 November 2007, 3:00pm

Dr. Bob McElrath, CERN
Dark Matter: From Cosmos to Collider
Location: Physics 135

A major focus in high-energy particle physics is to discover the nature of Dark Matter by directly producing it in a collider. Proving the particle dark matter hypothesis requires a three-pronged approach, first to discover its effects in the cosmos, second to discover that there exists a particle with the correct properties to be dark matter, and third to directly detect these particles with cosmological origin hitting detectors on earth. Purely gravitational observations cannot distinguish particle dark matter from much larger non-particle objects, such as small black holes. Direct detection does not have very good sensitivity to the dark matter mass or couplings. Colliders cannot prove that the new particle they produce is stable or the same as one in the cosmos. It is only by a coincidence in these three classes of observations that we can prove dark matter. I will describe the theories capable of solving this problem and some current efforts and evidence, with a focus on the collider measurements that have been made at low energy colliders such as BES in China, Belle in Japan, and CLEO at Cornell, on-going searches at the Fermilab Tevatron in Chicago, and upcoming searches at the CERN LHC in Geneva.

Friday, 30 November 2007, 3:00pm

Dr. habil. Marius Schmidt, Asst. Professor, University of Wisconsin-Milwaukee
Movies and Things: Structures and Kinetics from Time-Resolved Macromolecular X-ray Crystallography
Location: Physics 135

X-ray crystallography is the method of choice to determine the structures of biological macro-molecules. Traditionally, structure analysis is a static method due to the lack of time-resolution. The progress of chemical reactions within or catalyzed by these macro-molecules cannot be followed. However, time-resolved crystallography is a tool that allows just this. This talk will guide through the processes of data collection and, more importantly, data analysis. It will be shown how the structures of the intermediate states, as well as the chemical kinetic mechanism of the reaction, can be extracted from the time-resolved crystallographic data. This will lead to an unprecedented understanding of catalysis in general.

Friday, 7 December 2007, 3:00pm

Dr. Zhong Ren, Univ. of Chicago, The Center for Advanced Radiation Sources (CARS)
An Introduction to Laue Diffraction on the New 14-ID Beamline of BioCARS
Location: Physics 135

The 14-ID beamline of BioCARS at the Advanced Photon Source has been upgraded to take X-ray beams from two colinear undulators. The enormous flux of pink X-rays, and flux density with the also upgraded focusing capability, opens a variety of new opportunities to study fast and ultrafast processes in crystals, fibers and solutions by Laue diffraction and small/wide angle scattering techniques. Several previous examples of Laue diffraction will be presented. Newly proposed projects that require ultrafast data collection will be discussed. BioCARS invites new proposals to take advantages of the upgraded capabilities.