Friday, 14 December 2012
Connecting Subdiffusive Transport and Lateral Structure in Lipid Bilayers
Dr. Edward Lyman, Assistant Professor, Dept. of Physics and Astronomy & Dept. of Chemistry and Biochemistry, University of Delaware
Even apparently simple mixtures of lipids present interesting surprises. For example, a mixture of a high melting temperature lipid, low melting temperature lipid, and cholesterol displays a region of two-phase coexistence, where the two phases are both liquids, but distinguished by their composition. The existence of such coexisting liquid domains in live cell membranes has long been hotly debated, in part because the lengthscale of the putative domains is apparently too small for direct observation by light microscopy.
I will discuss our efforts to understand liquid-liquid coexistence in bilayers using “Anton,” a special purpose supercomputer built for performing molecular dynamics simulations. Providing access to unprecedented simulation timescales, data from Anton enables a direct link between the nature of lipid transport and the existence of composition heterogeneities. We find that simple two-phase coexistence is not sufficient to explain recent experimental observation of lipid transport on nanometer lengthscales.
Friday, 30 November 2012
The Successful Hunt for the Third and Final Neutrino Mixing Angle, θ13; and Implications for the Future
Dr. Thomas J. Weiler, Professor, Dept. of Physics and Astronomy, Vanderbilt University
Neutrinos are the second most abundant particle in the Universe, after photons. About 100 billion neutrinos from the Sun pass through each human fingernail every second, as well as another 100 billion relic neutrinos (“Bing Bang leftovers”) every second. And yet neutrinos were postulated to exist only in 1930, and only proven to exist 26 years later, in 1956 (one year AED – after Einstein's death). The past forty years have given us neutrino beams, neutrino scattering results, and differentiation of three neutrino types (and Nobel prizes, and my PhD dissertation); the past twenty years have offered a plethora of successful experiments showing that neutrinos “oscillate” among their three types (more Nobel prizes).
From oscillation experiments, one hopes to obtain five parameters that describe neutrinos and their Lagrangian — the three Euler-like angles that mix the neutrino types, a CP-violating phase of quantum mechanical origin, and the ordering parameter of the mass-eigenstates. Two of the three mixing angles have been known for some years now, but the coveted third angle, θ13, was precisely determined and definitively shown to be nonzero just this past summer. Its value is surprisingly large, thereby continuing the neutrino tradition of delivering surprises. Large θ13 greatly expands the reach of future neutrino experiments. Large θ13 also complicates the modeling of neutrino masses and mixings. In my colloquium, I will explain some of these developments.
Friday, 16 November 2012
The New Particle
Dr. Steve Nahn, Associate Professor, Department of Physics, MIT
For about 40 years, there has been a simple proposal on the table for how to maintain a working theory of fundamental particle physics while reconciling some glaring inconsistencies between theory and experiment, chiefly why fundamental particles have mass. The promise of the LHC is that further experimental scrutiny can be brought to bear on this question, leading to a decisive “go-no go” resolution, and indeed, in July of this year a major step was taken in the process with the discovery of a new particle which seems to fit the bill.
This talk will try to explain why we need such a particle, how one goes about finding it, and what is the current status of the evidence for both its existence and whether or not it is the one we've been waiting for.
Wednesday, 14 November 2012
Investigation of the Quaternary Structure of an ABC Transporter in Living Cells Using Spectrally Resolved Resonance Energy Transfer
Deo Raj Singh, PhD Candidate, UWM Dept. of Physics
A method for Förster Resonance Energy Transfer (RET) imaging of living cells recently introduced by our group permits determination of the quaternary structure of protein complexes from the number and relative disposition of peaks in the distribution of FRET efficiencies across FRET image pixels. Unfortunately, when the proteins of interest form rapidly dissociating/associating oligomers, the FRET efficiency distributions exhibit few or even single peaks, causing severe underestimation of the oligomer size.
In this talk I will describe a FRET-based method that circumvents this problem and allows one to probe the stoichiometry and structure of dynamic protein complexes in living cells. We use this method to determine the quaternary structure of an ABC transporter complex involved in the export of lipopolysaccharides to the surface of the pathogenic bacterium Pseudomonas aeruginosa. Lipopolysaccharide (LPS) is one of the most important virulence factors that are responsible for the pathogenicity of this bacterium. An understanding of the structure and behavior of this ABC transporter will help develop antibiotics targeting the biosynthesis of the A-band LPS endotoxin.
Friday, 9 November 2012
Quantum Mechanics for the Universe
Dr. James Hartle, Research Professor, Department of Physics, Univ. of California, Santa Barbara
Familiar textbook quantum mechanics must be generalized for application to cosmology. In a theory of the whole thing there can be no fundamental division into observer and observed. Measurements and observers cannot be fundamental notions in a theory that seeks to describe the early universe where neither existed. And in fundamental formulation of quantum mechanics, there is no reason for a `classical world' to exist in all circumstances.
This talk will describe at an introductory level the nearly 30 year old decoherent (or consistent) histories formulation of quantum theory that is applicable to cosmology. We will begin with models in which gross quantum fluctuations in the geometry of spacetime (quantum gravity) can be neglected, but then describe the further generalizations necessary to include quantum spacetime at the semiclassical level to give a quantum theory for quantum cosmology.
Friday, 2 November 2012
Fiber Diffraction from Biopolymers
Dr. Gerald Stubbs, Professor of Biological Sciences, Vanderbilt University
Many biopolymers have a natural tendency to form filaments. Even if crystals can be grown, the molecular interactions rarely correspond to the interactions in the biological filaments. Crystallo-graphy is therefore of limited use in studies of these assemblies. Fiber diffraction, in contrast, can be a powerful technique for determining their structural details. Fiber diffraction has been used to study biopolymers ranging from simple polypeptides, polynucleotides, and polysaccharides to amyloids, filamentous viruses, cytoskeletal filaments, and larger assemblies such as muscle fibers.
Experimentally, molecular filaments can be assembled into fibers in which the filaments are approximately parallel, but are randomly oriented about the fiber axis. In consequence, the diffraction pattern is cylindrically averaged, and the number of data is considerably less than from a single crystal. Methods have been developed to determine structures objectively from fiber diffraction patterns, and to refine these structures or initial models derived from other sources, for example high resolution cryo-electron microscopy, crystal structures of fragments of the fiber, or solid state NMR analysis.
We have applied these methods to the structure determination of a variety of filamentous assemblies, particularly amyloids and filamentous plant viruses. The structure of the rigid rod-shaped tobacco mosaic virus at 2.9 Å resolution was the first to be determined; more recently, we have been looking at flexible filamentous plant viruses and amyloids including the Alzheimer’s disease-associated Aβ and infectious mammalian prions, misfolded proteins associated with Creutzfeldt-Jakob disease and its variant, mad cow disease.
Friday, 26 October 2012
IceCube: Particle Astrophysics with High Energy Neutrinos
Dr. Francis Halzen, Professor, Dept. of Physics Principle Investigator, IceCube Research Center, UW-Madison
Construction and commissioning of the cubic-kilometer IceCube neutrino detector and its low energy extension DeepCore have been completed. The instrument detects neutrinos over a wide energy range: from 10 GeV atmospheric neutrinos to 1010 GeV cosmogenic neutrinos.
We will discuss initial results based on a subsample of the more than 300,000 neutrino events recorded during construction. We will emphasize the first measurement of the high-energy atmospheric neutrino spectrum, the search for the still enigmatic sources of the Galactic and extragalactic cosmic rays and for the particle nature of dark matter.
Friday, 12 October 2012
Studying the Evolution of Stars (and their Planets) via Kepler Asteroseismology
Dr. Steve Kawaler, Professor of Physics and Astronomy, Dept. of Physics and Astronomy, Iowa State University
By exploiting subtle brightness variations of stars exposed by precision photometry from space, we are using the tools of seismology to probe, in detail, the interiors of a wide variety of stars. For example, asteroseismology has proven to be a revealing tool aiding our understanding of what happens to stars like our Sun after they ignite helium in their cores.
In particular, data obtained by the Kepler spacecraft from stars that have survived the core helium flash — hot subdwarf B (sdB) stars and white dwarfs — are particularly helpful. Time-series photometry has also revealed strong evidence for planets around some sdB stars, raising fascinating questions about the fate of planetary systems.
In this talk I'll describe the tools of asteroseismology, and what they have revealed about the future evolution of our Sun.
Friday, 5 October 2012
Evidence of Nanocrystalline Semiconducting Graphene Monoxide during Thermal Reduction of Graphene Oxide in Vacuum
Professor Carol Hirschmugl, UWM Dept. of Physics
As silicon-based electronics are reaching the nanosize limits of the semiconductor roadmap, carbon-based nanoelectronics has become a rapidly growing field, with great interest in tuning the properties of carbon-based materials. Chemical functionalization is a proposed route, but syntheses of graphene oxide (G-O) produce disordered, nonstoichiometric materials with poor electronic properties. We report synthesis of an ordered, stoichiometric, solid-state carbon oxide that has never been observed in nature and coexists with graphene. Formation of this material, graphene monoxide (GMO), is achieved by annealing multilayered G-O. A combination of transmission electron microscopy and infrared microspectroscopy have provided critical experimental evidence to identify the novel structure. These results indicate that the resulting thermally reduced G-O (TRG-O) consists of a two-dimensional nanocrystalline phase segregation: unoxidized graphitic regions are separated from highly oxidized regions of GMO. GMO has a quasi-hexagonal unit cell, an unusually high 1:1 O:C ratio, and a calculated direct band gap of approximately 0.9 eV.
Thursday, 27 September 2012 — 11:30 AM-1:00 PM, Chemistry – Room 123
Variations in Reactivity on Different Crystallographic Orientations of Cerium Oxide Thin Films
Dr. David R. Mullins, Oak Ridge National Laboratory, Oak Ridge, TN 37831
Cerium oxide is a principal component in many heterogeneous catalytic processes. One of its key characteristics is the ability to provide or remove oxygen in chemical reactions. The different crystallographic faces of ceria present significantly different surface structures and compositions that may alter the catalytic reactivity. The structure and composition determine the number of coordination vacancies surrounding surface atoms, the availability of adsorption sites, the spacing between adsorption sites and the ability to remove O from the surface.
To investigate the role of surface orientation on reactivity, CeO2 films were grown with two different orientations. CeO2(100) films were grown ex situ by pulsed laser deposition on Nb-doped SrTiO3(100). The structure was characterized by RHEED, XRD and reflectometry. CeO2(111) films were grown in situ by thermal deposition of Ce metal onto Ru(0001) in an oxygen atmosphere. The structure of these films has been studied by LEED and STM. Attempts to grow CeO2(100) in situ by physical vapor deposition on Pt(100) and Pd(100) failed due to preferential growth of CeO2(111) on these supports.
The chemical reactivity was characterized by the adsorption and decomposition of various molecules such as alcohols, aldehydes and organic acids. Reaction products were monitored by TPD and surface intermediates were determined by soft x-ray photoelectron spectroscopy and soft x-ray absorption. In general the CeO2(100) surface was found to be more active, i.e. molecules adsorbed more readily and reacted to form new products, especially on a fully oxidized substrate. However the CeO2(100) surface was less selective with a greater propensity to produce CO, CO2 and water as products. The differences in chemical reactivity are discussed in light of possible structural terminations of the two surfaces.
Friday, 21 September 2012
Mesoscale Optics: Sensing and Action
Dr. Aristide Dogariu, Professor of Optics, CREOL, The College of Optics and Photonics, University of Central Florida
Harnessing light at scales comparable with the wavelength offers unique possibilities for sensing material properties and controlling the mechanical action of light.
We will review both passive and active applications of controlling the coherence and polarization properties of electromagnetic fields at these scales.
Wednesday, 18 July 2012
Unexplored Avenues of Human Skin: Reading personal stress in the Sub-THz frequency range
Dr. Yuri Feldman, Professor, Dept. of Applied Physics, The Hebrew University of Jerusalem
The coiled structure of the tips of the sweat ducts embedded in the epidermis of human skin has given rise to the supposition that at sub-THz frequencies the response of the ducts should be similar to low Q helical antennas. As such, this response should reflect the activity of the perspiration system, governed by the Sympathetic Nerve System.
We show that indeed the temporal behavior of the reflection coefficient at sub THz frequencies is highly correlated to the temporal behavior of the blood pressure, the pulse rate and other physiological parameters in response to physical, mental and emotional stresses imposed on the subject.
Friday, 6 July 2012
The Neutron-Star Equation of State and Gravitational Waves from Compact Binaries
Benjamin Lackey, PhD Candidate, UWM Dept. of Physics
The equation of state (EOS) of matter above nuclear density is currently uncertain by an order of magnitude. Fortunately, neutron stars provide an ideal laboratory for studying high density matter. In order to systematize the study of the EOS, we introduce a parametrized high-density EOS that accurately fits theoretical candidate EOSs, and we determine the ability of several recent and near-future electromagnetic observations to constrain the parameter space of our EOS.
In addition to electromagnetic observations, binary neutron star (BNS) and black hole-neutron star (BHNS) coalescence events observed with gravitational-wave detectors offer the potential to dramatically improve our understanding of the EOS. Information about the EOS is encoded in the waveform through tidal interactions, and for BNS systems, the inspiral waveform depends on the EOS through a single parameter called the tidal deformability. Using recent numerical BHNS simulations we find that the entire BHNS inspiral-merger-ringdown waveform also depends on the EOS exclusively through the same tidal deformability parameter. Using these waveforms, we examine the ability of second generation detectors now in construction and planned third generation detectors to extract information about the EOS.
Friday, 4 May 2012
Nanopores: Portals to the Molecular World
Dr. Jason R. Dwyer, Assistant Professor, Dept. of Chemistry, University of Rhode Island
Single-molecule sensing is a powerful approach for fundamental biophysical investigation and for application. Nanopores are the foundation of a new class of sensing devices and techniques that permit the direct sensing of single molecules without the need to label of the molecules. A nanopore is simply a nanometer-diameter hole in an insulating membrane that is used to divide an electrochemical cell in half. The voltage-driven passage of a single molecule through the nanopore will perturb the flow of ions through the pore and may consequently be detected as a change in the measured current.
This resistive-pulse sensing scheme provides single-molecule sensitivity and allows, in principle, characterization of the molecule’s physicochemical properties. Nanopore force spectroscopy (NFS), an analogue of atomic force microscopy, can dramatically augment the performance of nanopore sensing by providing a direct characterization of molecular binding energies. This capability, coupled with the robustness and ease-of-use of this all-electronic single-molecule sensor will be discussed in the context of a clinically-oriented genotyping assay.
Friday, 27 April 2012
Testing the No-Hair Theorem with Astrophysical Black Holes
Dimitrios Psaltis, Assoc. Professor, Astronomy Dept., University of Arizona
The Kerr spacetime of spinning black holes is one of the most intriguing predictions of Einstein's theory of general relativity. The special role this spacetime plays in the theory of gravity is encapsulated in the no-hair theorem, which states that the Kerr metric is the only realistic black-hole solution of the vacuum field equations. Recent and anticipated advances in the observations of black holes throughout the electromagnetic spectrum have secured our understanding of their basic properties while opening up new opportunities for devising tests of the Kerr metric.
In this talk, I will show how imaging and dynamical observations of accreting black-holes with current and future instruments will lead to the first direct test of the no-hair theorem with an astrophysical object. I will also discuss the current state of the Event Horizon Telescope, which will obtain, in the near future, the first horizon-scale image of the black hole in the center of the Milky Way.
Friday, 13 April 2012
Feedback and Galaxy Formation
Dr. Norm Murray, Director — CITA (Canadian Institute for Theoretical Astrophysics
Feedback from young stars plays a critical role in shaping the galaxy mass function, particularly at the low mass end, while feedback from supermassive black holes appears to shape the high mass end, statements supported by both numerical and semi-analytic models of galaxy formation. However, the exact form of the feedback is not certain.
I will describe recent work shedding light on this problem. First I will describe three dimensional radiative magnetohydrodynamics calculations of the effects of young stars and supernovae on giant molecular clouds, showing that star formation is rapid, but that feedback halts star formation when ~5-10% of the cloud is turned into stars; supernovae play only a minor role. Next I will describe high resolution (~1 parsec) SPH simulations of star forming galaxies employing momentum feedback from young stars, as well as heating from supernovae, O star winds, and HII regions, showing that all these forms of feedback have a role to play, with different forms of feedback coming to the fore in different galaxies. These simulations naturally produce galactic scale superwinds, with mass loss rates from 1-10 times the star formation rate, exactly what is needed to explain the low mass end of the galaxy mass function. Finally, I will briefly describe recent results on quasar feedback, including observational constraints one the launching mechanism of BAL winds, one of the more promising forms of “quasar mode” feedback.
Thursday, 5 April 2012
First-principles Studies of Polar Oxide Surfaces and Interfaces
Kanupriya Pande, PhD Candidate, UWM Dept. of Physics
Ultrathin oxide films have a variety of applications in modern electronics, devices for energy production, catalytic and complex functional materials.
In this talk I will describe a systematic first-principles study of the homo-epitaxial growth of MgO films on self-standing unreconstructed non-polar MgO(001) and polar MgO(111) slabs. Epitaxial growth is an example of a quasi-equilibrium dynamical process where the driving force for growth is the variation of constituent chemical potentials as a function of the instantaneous surface stoichiometry. The inherent difference in the surface structure of MgO(001) and MgO(111) results in significantly different thermodynamic growth pathways.
In the second part of the talk I will show results on the polar interface between MgO and Fe2O3 (hematite), focusing on the effects of substrate polarity on the trends in atomic and electronic structure at the interface. Hematite films show large structural relaxations and changes in the stacking of Fe and O planes that result in the formation of Fe2jFeO3 stackings which do not exist in any of the naturally occurring bulk iron oxides.
Friday, 30 March 2012
Dynamics of Soft Matter
Dr. Alexei Sokolov, Governor's Chair and Prof. of Chemistry & Physics, Univ. of Tennessee, and Chemical Sciences Div.; Oak Ridge National Lab
Broadly defined Soft Materials include polymers and glass-forming liquids, liquid crystals and colloids, biological and many other systems. Research in the field of Soft Matter experiences an explosive growth in recent years due to the great potential these materials offer for various advanced applications from energy to bio-medical fields. This talk emphasizes the importance of molecular motions (dynamics) for basic properties of Soft Materials. Significant parts of the talk will focus on decoupling phenomena in dynamics of Soft Materials. One example of this decoupling is the failure of the classical Debye-Stokes-Einstein relationship between molecular diffusion and viscosity, or structural relaxation.
The decoupling phenomena in dynamics of molecular, polymeric and biological systems are described and their general mechanism is discussed. The final part of the talk presents a discovery of very strong decoupling of ionic conductivity from structural relaxation in polymers. We discuss possible microscopic mechanisms of this strong decoupling and its potential for design of solid polymer electrolytes for battery applications.
Friday, 16 March 2012
8% Budget Cuts and Public Peer Review: Science Policy in America
Dr. Tyler J. Gembo, Government Relations Specialist, APS Physics
Basic science, as a majority federally funded initiative, is embedded in politics in that it informs public debate by providing data to lawmakers. Those same lawmakers, informed by science, then appropriate money into areas of science deemed to be most worthwhile, creating a feedback loop. Moreover, the policies enacted impact the method by which science is done in meaningful ways, such as anonymous versus public peer review during the grant approval process at federally funded agencies.
The policy set forth and the budget appropriated by Congress therefore set constraints that are very real concerns for scientists who are funded by federal agencies such as NSF, DOE, NASA Science, etc. Physicists can influence this feedback loop in a number of ways. Here we examine a few of the current issues in science policy such as how the Budget Control Act will affect the future of funding for basic research and the effect that the GRANT Act would have on peer review. We then shed light on how physicists can effectively weigh in on policy issues with help from APS and the positive impact that such public outreach has on an academic career.
Friday, 9 March 2012
A Different View of Physics
Dr. Basab B. Dasgupta, PhD Graduate of UWM, Former Asst. Prof., of Astronomy and Physics, Marquette University, Former VP at Sony Electronics (San Diego)
Contrary to popular belief, physics does not really explain how the material objects function in this universe. It is really just an analytical tool – brilliantly elegant and highly mathematical – which looks at thousands of physical phenomena which cannot be explained and reduces them by a cause-effect logic into a more compact set of a much smaller number of phenomena which still cannot be explained. We simply take them for granted and call them the “laws of physics.” Physics education does not lead to useful invention either. It is the design engineers who make our lives comfortable and convenient by inventing things and they neither understand physics nor care much for it. Furthermore, physics education shapes our personality by making our thought process analytical, disciplined and logical and by suppressing our spontaneity and creative spirit in other areas such as art, music and performing arts.
However, physics education allows one to develop the ability to analyze a complex situation and get to the “root causes” which can then be addressed in a controllable way. This ability is invaluable whether one is managing a large manufacturing operation or a deep personal crisis. It is also quite possible that the laws of physics are just a small subset of much more generalized principles which govern not only the motion of material objects but also physical and emotional behaviors of living beings. Perhaps these should be the next frontier in the study of physics.
I will present my views on all of these, from personal experiences, with a variety of examples, in a story-telling manner.
Friday, 2 March 2012
Neutron Star Radii and Masses
Dr. Feryal Ozel, Asst. Professor of Astronomy and Physics, University of Arizona
Neutron stars offer the unique possibility of probing the equation of state of cold, ultradense matter.
Understanding the properties of the neutron star interior is also important for predicting the observational appearance of short gamma-ray bursts, the end stages of neutron star coalescence, and the outcomes of supernova explosions.
I will present the recent measurements of neutron star radii and masses. I will show how the combination of the tightly constrained radii and the discovery of a 2 solar mass pulsar allows for the first astrophysical inference of the pressure of cold matter above nuclear saturation density.
I will discuss the implications of this measurement for nuclear theory and astrophysics.
Friday, 17 February 2012
Chemical analysis with sub-Å resolution: The power and challenges of aberration-corrected scanning transmission electron microscopy
Dr. Robert F. Klie, Nanoscale Physics Group, Department of Physics, University of IL – Chicago
The last few years have seen a paradigm change in (scanning) transmission electron microscopy, (S)TEM, with unprecedented improvements in both spatial and spectroscopic resolution being realized by aberration correctors, cold-field emission guns and monochromators. The spatial resolution now extends to the sub-angstrom level, while the spectroscopic resolution has reached the sub-100 meV regime. In-situ stages have further extended the temperature range where atomic-resolution can be achieved between 10 K and 1000 K. These instrumentation developments have brought notable successes in materials analysis, in particular for interfacial, catalysis and thin-film studies. However, they have also challenged the established experimental protocols and our fundamental understanding of both imaging and spectroscopy in the STEM.
In this presentation, examples of where the new instrumentation has been successfully used to address materials physics issues in nanoscale systems will be described, including magnetic transitions in oxide thin films, charge transfer in thermoelectric oxides, and promoter diffusion in heterogeneous nano-catalysts. Furthermore, the challenges associated with operating these new STEMs for reliable quantitative imaging and spectroscopy will be discussed. Finally, I will present a perspective on the future developments in STEM analysis.
Friday, 27 January 2012
Plant Phenology: From Individuals to Continental-Scale
Dr. Mark D. Schwartz, Distinguished Professor and Chair, Dept. of Geography, University of Wisconsin – Milwaukee
Plant phenology, the study of observable life‐cycle events (such as bud‐break, first bloom, leaf coloring), driven by weather and climate, offers multiple opportunities to better understand the functioning of Earth’s terrestrial biosphere at multiple scales, especially in light of on‐going global climate change.
This talk will provide an overview of past and current research exploring the interactions between terrestrial plants and the lower atmosphere, by linking phenological events with carbon and energy flux measurements, as well as satellite‐derived surface reflectance. The foundation for future research advances in this area has been laid through the recent creation of a national‐scale phenological observation network, the USA National Phenology Network (www.usanpn.org, USA‐NPN) which facilitates phenological data collection and sharing across the country and around the globe.