Physics Colloquia - 2013

The Physics Department colloquia are usually on Friday afternoons at 3 PM in Room 135. Coffee and cookies are served at 2:45pm in the same room. Anyone is welcome to attend.

Physics Colloquia Titles and Speakers

Friday, 13 December 2013

Ultrafast X-ray Diffraction at the Linac Coherent Light Source

Dr. Uwe Weierstall, Research Professor, Dept. of Physics, Arizona State Univ.

The advent of the first hard X-ray free electron laser has provided a new tool for structural biology that may revolutionize the field. It allows protein structures to be determined from protein nanocrystals and may in the future even allow structure determination from single molecules. Biological molecules and nanocrystals are injected into femtosecond high intensity X-ray pulses and diffraction patterns from thousands of particles are recorded with essentially no radiation damage, before the sample is destroyed. After presenting the general method and the latest results, different sample injection methods and devices currently in use at the LCLS will be discussed in detail.

Friday, 6 December 2013

Tides in Coalescing Neutron Star Binaries

Dr. Nevin Weinberg, Asst. Professor, Dept. of Physics, MIT

Within the next few years, advanced versions of ground-based gravitational wave observatories such as LIGO and Virgo are expected to detect the first gravitational waves from the merger of neutrpn star binaries. Tidal interactions in such systems extract energy from the orbit and, at some level, modify the gravitational wave signal. Previous studies found that tidal effects are probably too small to be detected with advanced LIGO and Virgo. However, these studies all assumed that the tide can be treated as a linear perturbation to the star.

In this talk I will show that the linear approximation is invalid even during the early stages of inspiral and that nonlinear effects might become important around the time the binary first enters LIGO's bandpass. Although the precise influence of nonlinear effects is not yet well constrained, I will show that they may significantly modify the gravitational wave signal and electromagnetic emission from coelescing neutron star binaries.

Friday, 15 November 2013

Protein Association in Living Cells Using FRET Spectrometry: Application to G-Protein Coupled Receptors

Suparna Patowary, UWM Physics PhD Candidate

G-protein coupled receptors (GPCR) form a superfamily of cell surface signaling proteins that constitute one of the largest drug targets in the pharmaceutical industry. We used a novel Fluorescence (Förster) Resonance Energy Transfer (FRET) spectrometric method developed in our lab to study the quaternary structure of muscarinic M3 acetylcholine receptor (M3R), a GPCR that has elicited significant interest in recent years. The method was tested on FRET standards consisting of linked fluorescent proteins that form dimeric, trimeric, and tetrameric combinations of donors and acceptors of energy located in the cytoplasm or at the plasma membrane.

Our findings showed that the M3R exists as a stable dimer at the plasma membrane though a large fraction of them interact dynamically to form tetramers without forming trimers, pentamers or any other higher order oligomers. Based on this, we propose a conceptual framework that reconciles the conflicting views on the quaternary structure of GPCRs.

Friday, 8 November 2013

How to Pursue a Career in Physics Knowing Only One Equation

Dr. Gerald R. Harp, Director, Center for SETI Research, SETI Institute

Harp's physics career began as an undergraduate in 1980 at the University of Wisconsin-Milwaukee. He has pursued a variety of topics in experimental physics while employing just one tool, the Fourier Transform. The Fourier Transform (FT) can re-express any function or fields in spacetime coordinates in terms of its frequency spectrum. Experience shows that no matter where you go or what you do, you can always look like a hero with application of this single equation. It is not necessary to know this equation beforehand to enjoy this talk.

In the second half of the presentation, Harp discusses recent work in the Search for Extraterrestrial Intelligence (SETI). The SETI Institute is a nonprofit organization devoted to the study of the formation and distribution of life in the universe, performing all kinds of astrobiology research melding biology, chemistry, physics, astronomy and cosmology into a massive program to understand life as we know it. One component of this research attempts to use radio telescopes to discover indications of technology outside the solar system. SETI is difficult and progresses slowly, but at present it is the only means by which humans may discover exo-life for at least the next 10-15 years. Just long enough to safely land Dr. Harp's career.

Thursday, 7 November 2013

Surface X-ray Diffraction Study of Polar Oxide Surfaces and Interfaces

Wei Han, UWM Physics PhD Candidate

An atomic scale study of surface/interface structure is required to properly understand physical and chemical phenomena such as crystal growth, lubrication and electrochemistry. The stability of polar oxide surface has long been an interesting question. A bulk-terminated polar oxide surface comprises alternating layers of opposite charges, thus resulting in diverging surface energies. In order to reduce the surface energy, various reconstruction-stabilized MgO (111) surfaces have been reported experimentally.1 However, the atomic structure of the MgO (111) (√3×√3) R30° reconstructed surface remains unclear. Using a third-generation X-ray source is one of the feasible methodologies to probe such a system due to its increase of sensitivity on the interface layer.

Surface X-ray diffraction (SXRD) experiments were performed for the MgO (111) (√3×√3) R30° reconstructed surface at Advanced Photon Source, Argonne National Laboratory. Crystal truncation rod (CTR) and super structure rod (SSR) measurements were acquired in both the absence and presence of a thin layer of water, obtained by compressing the bulk water layer with a thin Kapton sheet.

A differential evolution algorithm, GenX, was used to search for the appropriate atomic model of reconstructed structure. Some reasonable models are presented and discussed with quantitative calculation of optimizing parameters (R factor and λ2). Preliminary SXRD results of the dry surface and solid-liquid interface are compared. This determination will shed light on future understanding of the properties of polar oxide surface/interface.

Friday, 1 November 2013

Neutron Star Masses and Basic Physics Consequences

Dr. Scott Ransom, National Radio Astronomy Observatory (Charlottesville, VA)

Over the past several years, astrophysical observations of neutron stars using X-rays and radio wavelengths have made significant progress towards determining the Equation of State of neutron star matter. The discovery of several interesting new pulsars as well as improved instrumentation has finally allowed us to start measuring the masses of millisecond pulsars. These systems have had potentially substantial amounts of mass accreted onto them during the "recycling" process and are likely more massive, on average, than the "canonical" 1.4 Msun neutron stars found in Hulse-Taylor-like double neutron star binaries. The so-called Shariro Delay has been used to make very precise measurements of 1.67 and 1.97 Msun neutron stars in the past three years. These systems strongly constrain the equation of state of nuclear matter and a variety of other topics in physics/astrophysics.

Finally, I'll show that there is good potential for more measurements in the near future, including the mass of a fantastic new millisecond pulsar in a hierarchical triple system.

Tuesday, 22 October 2013

Structure and Function of Proteins Investigated by Crystallographic and Spectroscopic Time-Resolved Techniques

Namtra Purwar, UWM Physics PhD Candidate

Biomolecules play an essential role in perpetuating the necessary functions for life. The goal is to contribute an understanding of how biological systems work on the molecular level. We used two biological systems - beef liver catalase (BLC) and photoactive yellow protein (PYP). BLC is a metalloprotein that protects living cells from the harmful effects of reactive oxygen species by converting H2O2 into water and oxygen. By binding nitric oxide (NO) to the catalase, a complex was generated that mimics the Cat-H2O2 adduct (a crucial intermediate in the reaction promoted by the catalase).

Enzyme kinetics is affected by additional parameters such as temperature and pH. In crystallography, the absorbed X-ray dose may impair protein function in addition. To address the effect of these parameters, we performed time-resolved crystallographic experiments on a model system, PYP. Results from the dose and the pH-dependent time-resolved crystallographic experiments show that it is imperative to carefully control the conditions under which the time-series are collected. With these considerations we investigated the effect of temperature on protein kinetics. Kinetic modeling yields entropy and enthalpy values at the barriers of the activation solely from the time-resolved crystallographic data. Results from time-resolved crystallography are corroborated by employing time-resolved absorption spectroscopy. For this, time-resolved absorption spectra on crystals and solution are collected by a fast micro-spectrophotometer custom-designed in our lab.

Friday, 25 October 2013

Galaxy Clusters in the Distant Universe

Professor Kim-Vy-Tran, Dept. of Physics and Astronomy, Texas A&M University

Galaxy clusters are the largest gravitationally bound systems in the universe and are extreme laboratories for studying the physics driving galaxy evolution as well a powerful test of cosmology.

Understanding how galaxies form and evolve in clusters continues to be a fundamental question in astronomy. I present results from an ongoing multi-wavelength study of galaxies in clusters at z>1.5 to track how they assemble their stars.

Friday, 18 October 2013

Probing Molecular Interactions with Time-Correlated Single Photon Counting Technique

Dr. Vladislav Shcheslavskiy, Senior Research Fellow, Becker & Hickl GmbH (Berlin)

To investigate molecular interactions in cells and subcellular structures, fluorescence markers are used which specifically link to protein structures. Staining the samples with different dyes and recording the fluorescence image reveals the cell structures via the different fluorescence specta and fluorescence lifetime of the dyes. Energy transfer between the dye molecules and the proteins changes the fluorescence quantum efficiency and thus the fluorescence lifetime. Due to the variation of the dye concentration these effects cannot be distinguished in simple intensity images. Therefore, recording time-resolved patterns of the full fluorescence decay functions rather than simple intensity imaging is required to investigate molecular interactions in biological systems.

I will present fluorescence lifetime imaging (FLIM) systems based on time-correlated single-photon counting (TCSPC) technique that have been developed at Becker & Hickl GMbH and discuss recent biologically-oriented research done in the company.

Friday, 11 October 2013

Mass Ejection and Electromagnetic Counterparts of Neutron-Star-Binary Mergers

Professor Masaru Shibata, Yukawa Institute for Theoretical Physics, Kyoto University

The mergers of binary neutron stars and black hole-neutron star binaries are among the promising sources of gravitational waves.

To confirm the future detection of gravitational waves and to determine the sky location of the event, it is quite important to find an electromagnetic counterpart of the gravitational-wave event. It has been suggested since the first paper by Li and Paczynski in 1998 that a strong electromagnetic signal could be generated by the materials ejected during the merger of the binaries.

I will present our latest numerical-relativity studies on the mass ejection and associated electromagnetic signal that can be generated by the radioactive decay of r-process elements.

Friday, 20 September 2013

In Search of Perfection: The quest for atomically flat silicon and the mechanism of silicon oxidation

Professor Melissa A. Hines, Dept. of Chemistry, Cornell University

Because of its technological importance, silicon oxidation has been studied intensely for decades; however, the disordered nature of the oxide makes this reaction notoriously difficult to understand. In this work, the oxidation reaction is coupled with a subsequent etching reaction, allowing oxidation to literally write an atomic-scale record of its reactivity into the etched surface -- a record that can be read with a scanning tunneling microscope (STM) and decoded into site-specific reaction rates, and thus chemical understanding, with the aid of simulations and spectroscopy.

This record overturns the long-standing and much-applied mechanism for the aqueous oxidation of the technologically important face of silicon, Si(100), and shows that the unusually high reactivity of a previously unrecognized surface species leads to a self-propagating etching reaction that produces near-atomically flat Si(100) in a beaker at room temperature -- a long-standing technological goal. These findings show that, contrary to expectation, the low-temperature oxidation of Si(100) is a highly site-specific reaction and suggests strategies for functionalization by low-temperature, solution-based reactions.

Friday, 13 September 2013

New Semiconductors for a Clean Energy Future: From Basic Properties to Computational Materials Design

Dr. Guenter Schneider, Asst. Prof., Dept. of Physics, Oregon State University

Computational condensed matter physics and materials science is moving rapidly from merely computing material properties and explaining experimental results to designing new materials 'in silico'. The trend to involve theory in the discovery of new materials stems from the desire to bring a more systematic approach to the frequently accidental nature of materials discovery. Nowhere is this trend stronger than in the search for new materials for energy conservation and storage. I will discuss the promise and challenge of computational materials design for the case of new semiconductors for photovoltaics. How accurately can theory predict optical and electronic properties as well as growth and synthesis? What design principles exist to guide the search for new semiconductors?

I will try to answer these questions using work from my research group on layered mixed anion compounds and various Copperchalcoginides as examples. These materials have potential applications in thin film solar cells and transparent electronics, which require the knowledge of optical, electronic, and interfacial properties, which in turn require increasingly complex calculations for ideal crystals, point defects, and extended interfaces.

Friday, 6 September 2013

Serial Femtosecond Protein Nanocrystallography Using a Free Electron Laser

Dr. Nadia Zatsepin, Arizona State University

The extremely intense, ultra-short X-ray pulses produced by X-ray free electron lasers (XFEL), such as the Linac Coherent Light Source at SLAC, have enabled studies of fully hydrated, room temperature biological samples such as viruses and protein crystals in the diffract-and-destroy regime. Serial femtosecond nanocrystallography (SFX) (Chapman et al. 2011), exploits these intense, femtosecond X-ray pulses (which terminate before the onset of radiation damage (Barty et al. 2012, Caleman et al. 2012)), instead of freezing, to yield high-resolution structures from protein nano/microcrystals in a biologically relevant environment. This facilitates dynamic, time-resolved studies such as pump-probe crystallography (Aquila et al. 2012).

In SFX, millions of nano/microcrystals are delivered in a micron-sized liquid jet across a pulsed XFEL beam, in vacuum (Weierstall et al. 2012) and a huge number of diffraction shots are obtained, each one (ideally) from an individual crystal. Using nanocrystals is particularly advantageous when the growth of crystals of sufficient size and quality for conventional crystallography proves impracticable. The capabilities of SFX have been demonstrated with (e.g.) a 1.9Å resolution structure of hen egg white lysozyme (HEWL) (Boutet et al., 2012), and a 2.1Å resolution structure of Trypanosoma brucei cysteine protease cathepsin B (TbCatB) (Redecke et al., 2012), and ~ 3Å resolution structure of a large membrane protein complex, Photosystem I (PSI). Time resolved (pump probe) studies are being developed on a number of systems, including PSI-Ferredoxin, and Photosystem II during its oxygen evolving cycle.

A significant limiting feature of SFX is wastage of precious biosamples and the need to collect a huge amount of data. This talk will also discuss a number of developments that are underway to mitigate this.

Friday, 9 August 2013

Exploration of W-WIMP Higgs Portal Model

Brian J. Vlcek, UWM Physics PhD Candidate

Recent dark matter direct detection research has found hints of a WIMP with mass in the range of 10 GeV reported by the DAMA/LIBRA, CoGeNT, CRESST, and CDMS collaborations, each of which report signals consistent with a dark matter particle of similar mass and an elastic scattering cross section with nucleons of 2X10-41 cm2.

Furthermore, it is observed that around the Galactic Center (GC), there exists a bright and spatially extended source of γ-ray emission peaking at energies of a few GeV, which the spectrum and morphology of this signal is consistent with one originating from dark matter annihilations with the location coincident with the location of the Fermi Bubbles. As well, Planck 2013 data, combined with H0 measurements, suggests an excess of relativistic degrees of freedom at the time of the CMB epoch then that of the standard model at the level of 2.3σ.

In this talk, we will review a minimal Higgs portal model suggested by Steven Weinberg and its ability to fit the observations above and LHC constraints.

Wednesday, 10 July 2013

Solving Virus Structures from XFEL (X-ray free-electron laser) Diffraction Patterns of Random Particle Orientations Using Angular Correlations of Intensities

Miraj Uddin, UWM Physics PhD Candidate

The world's first x-ray free electron laser (XFEL), the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC), is now creating x-ray pulses not only of unprecedented brilliance, but also of extremely short duration. Among the promised capabilities of this fourth-generation x-ray source is the ability to record diffraction patterns from individual bio-molecules and viruses. We developed a theory based on angular correlations of measured diffraction data to determine if the scattering is from an icosahedral particle. Based on the test an efficient algorithm can combine diffraction data from multiple shots of biomolecules random in orientation to construct a 3D real space image of the molecule. We successfully applied this method for simulated diffraction patterns from satellite tobacco necrosis virus (STNV) and constructed a real space image of STNV.

We have also analyzed experimental XFEL data from the icosahedral virus paramecium Bursaria chlorella virus (PBCV) and showed that the recovered radial function from the angular correlations has a pattern of an icosahedral particle.

So far, fiber diffraction is the primary method fro solving the structure of some of the most important biomolecules in in nature, such as dioxy-ribonucleic acid (DNA) and some of the helical viruses such as the tobacco mosaic virus (TMV), where helical molecules are aligned along a uniaxial direction to form a fibrous bundle. We have demonstrated that fiber diffraction can be obtained from XFEL diffraction-and-destroy single particle experiments, where the particles are exposed to the x-rays in completely random orientations, thus obviating the need for an experimental alignment mechanism, which is difficult to achieve due to the entropic tendency to disorder.

Friday, 31 May 2013

Unifying Driving Force for Stabilizing Polar Surfaces of ZnO

Dr. S. Y. (David) Tong, Nanostructure Institute for Energy & Environmental Research, Dept. of Physics, South University of Science & Technology of China (Shenzhen)

Experimental and theoretical evidence is presented to support a unified reconstruction model in which the bonding flexibility of Zn atoms provides the common driving force to stabilize the (0001) and (000-1) polar faces of ZnO crystals. Transition metal Zn has filled core shells and two 4s orbitals in the outermost shell. The relatively isotropic nature of the 4s orbitals of Zn allows a high degree of flexibility in bond coordination and bond angles. On these two reconstructed polar faces, a total of four novel bonding configurations involving surface Zn are identified. The reconstructions result in the loss of more Zn ions than O ions on the Zn-terminated face and vice versa on the O-terminated face. Surface O atoms bond at fcc sites on the two surfaces. Next to these fcc oxygen, the Zn ions are either 4-fold coordinated in a “tilted tetrahedral” structure on the Zn-polar face or 3-fold coordinated in two “T” structures on the O-polar face.

Friday, 10 May 2013

The Changing Landscape of Type Ia Supernova Progenitors

Dr. Ken Shen, Lawrence Berkeley National Lab

Type Ia supernovae (SNe Ia) have been made famous for their role in determining the accelerating expansion of the Universe, and yet, despite concerted effort, their progenitors have not been definitively identified. For decades, we have assumed that the "single degenerate" progenitor channel offers the most plausible explanation. However, evidence is mounting that this standard, textbook progenitor channel cannot be responsible for the bulk of SNe Ia.

I will outline the current status of the two alternative progenitor channels and highlight our recent work on the "double detonation" scenario. While relatively less studied, this progenitor channel is extremely promising and may pave the way for an understanding of how these cosmic standardizable candles are born.

Friday, 3 May 2013

Physics in Art: a course and a museum tour

Dr. Sudha Swaminathan, Associate Professor, Dept. of Physical and Earth Sciences, Worcester State University

As physics teachers, we would like to inform a wide audience about the scientific examination of objects that are part of their cultural heritage. Physics in Art is an interdisciplinary lab course for non-science majors with a focus on physical methods used in the analysis of art objects. Through examples from optics, multispectral imaging and nuclear physics applied to art conservation, we try to make modern physics accessible in a non-traditional context. Our students perform hands-on-experiments with optical elements, sample paintings and an infrared camera. They are taught how the elemental composition of coins can be obtained by x-ray fluorescence and neutron activation analysis. At the end of the course the students take a live tour of the Worcester Art Museum. They see how paintings and pieces of great historic and artistic value are analyzed using physical techniques similar to those discussed in the lecture and in the lab.

Friday, 26 April 2013

3-D Heterogeneous Reconstructions of Viruses by Single-Particle Cryo Electron Microscopy

Dr. Peter C. Doerschuk, Biomedical Engineering/Electrical and Computer Engineering, Cornell University

Even a small and simple virus particle is a large macromolecular complex that undergoes a variety of transformations as it progresses through its lifecycle. X-ray crystallography of virus crystals provides near-atomic spatial resolution at one point along the lifecycle and similar information at somewhat lower resolution is provided by single-particle cryo electron microscopy (cryo EM). Cryo EM experiments generally provide one approximately tomographic image of each of a thousand to a hundred thousand instances of the particle. A new computational approach to processing the cryo EM images will be described which takes advantage of the fact that many different particles are imaged to provide a statistical description of the particle's 3-D structure.

Use of this approach on Nudaurelia Capensis Omega Virus (NwV) will be described where the new approach appears able to provide kinetic information on maturation that was difficult to determine by current methods.

The work represents a collaboration with Qiu Wang (Cornell University), Yili Zheng (Purdue University, now Lawrence Berkeley National Laboratory), and John E. Johnson (The Scripps Research Institute).

Monday, 22 April 2013

First Light With The HAWC Gamma-Ray Observatory

Dr. Peter Stefan Westerhoff, UW-Madison Physics Department, Wisconsin IceCube Particle Astrophysics Center

The High Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) is currently under construction 4,100 m above sea level on the slope of Pico de Orizaba, Mexico. HAWC is a high-duty cycle, large field0of-view instrument capable of monitoring the gamma-ray sky between roughly 50 GeV and 100 TeV. The detector will be used to record both steady and transient gamma-ray sources and to provide an unbiased survey of the northern sky.

Upon completion, HAWC will comprise 300 large light-tight water tanks covering an area of 20,000 square meters. Each tank will be instrumented with four photomultipliers to detect particles from extensive air showers produced by gamma rays and cosmic rays. With HAWC, we hope to open a new window of astronomical observations and study the most energetic objects in the known universe. In this talk, we present the scientific case for HAWC, describe its design and sensitivity, and report on early results from data taken with the partially completed detector since October 2011.

Friday, 19 April 2013

Understanding Ferroelectric Order and Electron-Phonon Coupling with Fast Electrons

Dr. Peter Yimei Zhu, Department of Condensed Matter Physics, Brookhaven National Laboratory

In this presentation, I will review our recent work using atomic imaging and in-situ biasing on ferroelectric order and switching in hexagonal manganites such as ErMnO3, which is a multiferroic material. I will focus on the structure of topologically protected vortices and the associated ferroelectric domain-walls and their roles in ferroelectric reversal. I will also give a brief overview of our recent development of the 2.8MeV femtosecond electron diffraction apparatus. The principle and design of the pump-probe approach and its applications in strongly correlated electron systems will be discussed. Ultrafast electron microscopy provides a unique opportunity to the direct observations of behavior and interaction of electrons, spins and lattice and the mesoscopic transient phenomena including electronic inhomogeneity and phase separation in correlated oxides with unprecedented time and spatial resolution.

The author would like to acknowledge M. G. Han, V. Volkov, S. W. Cheong, P. F. Zhu, X. J. Wang, Y. Hidaka and J. Hill for their collaborations on the projects. The work was supported by US DOE/BES, under Contract No. DE-AC02-98CH10886.

Wednesday, 17 April 2013

Investigation of Adsorbate/Substrate Interaction on Reduced Graphene Oxide

Eric Mattson, UWM Physics PhD Candidate

Graphene oxide (GO) is a popular and inexpensive precursor to graphene-based materials. Chemical reduction of GO to yield reduced graphene oxide (RGO) is one route toward the production of ultrathin conducting films with structure and properties resembling those of graphene. Artifacts of the oxidation-reduction processes, however, result in characteristics that distinguish RGO from pristine graphene.

In this talk, I will discuss the nature of residual oxygen in RGO and how these functional groups participate in chemical processes on RGO surfaces and films. In particular, the adsorption of the industrially and environmentally relevant molecules NH3 and NO2 are considered due to the growing use of RGO in chemical sensors and catalyst supports. The NH3/RGO and NO2/RGO systems are investigated using in situ IR microspectroscopy to study the adsorbate bonding configurations and their interaction with the substrate.

Friday, 12 April 2013

Structural Analysis of Membrane Proteins in Infectious Diseases

Dr. Katerina Dörner, Center for Membrane Proteins in Infectious Diseases, MPID, Department of Chemistry and Biochemistry, Arizona State University

Membrane proteins are key players in infectious diseases and represent more than 60% of all drug targets. The Center for Membrane Proteins in Infectious Diseases (MPID) was established to perform crystallization and structure determination on viral, bacterial and human membrane proteins involved in pathogenesis.

The talk will give an overview about the center’s aims and activities, including the used biophysical methods. One selected topic of our work focuses on membrane proteins involved in iron acquisition in Francisella tularensis. F. tularensis is one of the most infectious pathogens known and causes tularemia, a disease of humans and animals. Iron is an important nutritional requirement for mammals and bacteria due to its conserved role in many essential metabolic processes. Iron uptake within the iron-limited host environment is likely to be critical for intracellular survival and pathogenesis of F. tularensis.

Wednesday, 10 April 2013

Growth and Structure of ZnO Thin Films on Polar (√3×√3)R30° Reconstructed and Unreconstructed MgO(111) Surfaces by Atomic Layer Deposition

Kallol Pradham, UWM Physics PhD Candidate

Polar heterointerfaces of MgO(111) and the II-VI semiconductor ZnO are of technological interest for transparent conducting applications. Growth and structure of thin films on polar surfaces can be different than on non-polar surfaces due to the large surface energy of polar surfaces. We have grown ZnO on unreconstructed MgO(111)-(1x1)-OH terminated and reconstructed MgO(111)-(√3x√3) R30° polar oxide surfaces using atomic layer deposition.

In this talk, I will describe how a homemade ultra high vacuum-interfaced viscous-flow atomic layer deposition (ALD) reactor with in-situ quartz crystal monitor was used to grow ZnO thin films on the MgO(111) substrates. This study indicates the crystal orientation during ALD ZnO growth depends not only on temperature but also on the surface terminations of the substrates.

Friday, 5 April 2013

Finding Radio Transients with the Murchison Widefield Array

Dr. David Kaplan, UWM Physics

Explorations of the radio sky in the time-domain are an exciting frontier in astrophysics, and one where new observational capabilities will open up new windows on the universe. As one of a new generation of widefield, low-frequency radio telescopes, the Murchison Widefield Array has enormous potential to conduct blind searches for radio transients. We are working to develop a common framework with the Australian Square Kilometer Array Pathfinder (ASKAP) Variables and Slow Transients (VAST) survey to allow real-time transient detection and characterization.

I will discuss the expected types of sources that we hope to discover with the full array and review some of the initial results from our 32-element testbed.

Friday, 29 March 2013

Advanced Photovoltaic (PV) Technologies: Challenges & Opportunities

Dr. Devendra K. Sadana, T. J. Watson Research Center, IBM, Yorktown Heights, NY

This presentation will give a high level overview of various PV technologies prevalent in the market today. Innovative approaches to enhance cell efficiency while reducing the material cost will be presented. A recent breakthrough in releasing Si, Ge, and III-V films of desired thickness by controlled spalling will be discussed. The application of controlled spalling in reducing substrate cost via its reuse as well as in enabling thin and flexible PV products will be emphasized. Finally, some remarks on future direction of the PV industry will be made.

Friday, 8 March 2013

The battle of Signal vs Noise, and how to tip the balance in your favor

Dr. James Holton, Lawrence Berkeley National Lab

The success or failure of any structure determination effort is dictated by the signal-to-noise ratio, so quantitative predictors of the both signal and noise on an absolute scale are needed for designing effective data collection strategies. There are three main classes of noise in the diffraction experiment: photon-counting error or “shot noise” which is proportional to the square root of the signal, noise that is independent of the signal such as detector read-out noise, and “fractional noise” that is proportional to the signal. This last class of error has many sources, including shutter jitter, incident beam flicker, sample vibration, detector calibration, and systematic errors such as the uncertainty in absorption correction factors and the uncorrectable “non-isomorphism” component of radiation damage.

Procedures for independently measuring all these sources of error on a given instrument are described, and it was found that the dominant source of error in measurements of weak spots is the background-scattered photons that fall into the spot area, but the dominant source of error for anomalous difference measurements is fractional noise, which is usually 2-3%. The elimination of background scattering would allow the use of crystal sizes small enough for radiation damage reduction by photoelectron escape to become practical, and reducing calibration error to less than ~0.2% could make things like native S-SAD phasing not just possible, but routinely accessible.

Friday, 8 February 2013

Towards a Theory for How Much it Rains: Understanding the response of the hydrological cycle to climate change

Dr. Paul O'Gorman, Victor P. Starr Associate Professor of Atmospheric Science, Department of Earth, Atmospheric, and Planetary Sciences, MIT

Simulations with climate models suggest there will be major changes in the hydrological cycle in response to climate warming. However, some important aspects of the hydrological cycle, such as the intensity distribution of precipitation, are difficult to adequately simulate in a global model.

In this talk I will discuss some simple physical theories for how the hydrological cycle responds to climate change, and how observations can be used to help constrain the expected response.