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Graduate Student Seminar Series,
2007-2008 |
Cosmic rays are the most energetic particles observed in the Universe. They include nuclei, electrons, the highest energy gamma-rays, and neutrinos, and originate from both local and extragalactic sources. The extraterrestrial origin of cosmic rays was identified in 1912 by Victor Hess using balloon-borne electrometers, and scientific ballooning continues to play a major role in cosmic ray research today. The Trans-Iron Galactic Element Recorder (TIGER) experiment has flown twice from McMurdo, Antarctica, in December 2001 and 2003, with a cumulative flight time of ~50 days. The TIGER instrument uses a combination of scintillation and Cherenkov detectors to measure particle charge (Z) and energy (0.3 to 10 GeV/nucleon in instrument). TIGER's measurements of the relative abundances of the ultra-heavy cosmic rays (greek characters30 < Z < 40) are the best to date, and provide insight into the likely source of the galactic cosmic rays. TModeling approaches for fast laser-induced self-organization of nanoscopic metal filmshe observed roughly equal abundances of Ga, Ge, and Se pose a challenge to current source and to nucleosynthesis models.
Noninvasive functional neuroimaging through such techniques as fMRI and PET has revolutionized the field of neuroscience. However, there are certain populations where these systems are inadequate. Their utility for clinical imaging is limited, as ICU patients can not be transported to fixed scanners. In addition, their ability to study crucial stages in brain development is restricted,since small children and infants must be sedated for these scans. An emerging method which solves some of these issues is diffuse optical tomography (DOT). DOT uses near infra-red light to measure the concentration of hemoglobin in the brain, relying on portable equipment and a wearable imaging cap, making the technique ideal for many analyzing many previously inacFebruary 1: Matthew Caudillcessible questions. However, DOT has been limited by low lateral resolution and low depth penetration. High-density DOT is a method to improve these limiting factors which maintaining simple instrumentation. The Culver Lab has been in the process of proving the performance of our new high-density DOT system within the visual cortex, an ideal area for study due to its detailed structure. These studies have shown that our imaging system has higher lateral resolution and better dynamic range than other DOT devices. Current studies are focusing on full retinotopic mapping, comparisons of these maps with the gold standard of fMRI, determining functional borders with DOT, and using subject-specific anatomic MRI information for more accurate light-modeling and image reconstruction.
Bayesian probability theory has been used in a wide variety of applications: from spam filters to estimating the mass of Saturn. The purpose of this talk is to explain the application of Bayes’ theorem to electroencephalographic (EEG) tracings of premature infants. Though EEG lacks the detail of imaging techniques such as MRI, its easy bedside use makes it a useful tool in clinical diagnoses. However, interpreting EEG tracings currently requires the use of specialists, of whom there are few. Implementing an automated system that characterizes EEG tracings would make information obtained in EEG recordings more accessible. The talk will give an overview of Bayesian probability theory as well as its specific application to this problem.
No transparent steel here. Rather, ‘metallic glass’ refers to a material whose atomic structure combines the durable properties of metals with the elasticity and fracture resistance of amorphous materials (glasses). As you might expect, getting Aluminum alloys to share the material properties of Tupperware requires careful finessing of many parameters, beginning with the basic alloy composition. In fact, adding 0.5% of a transition metal to the particular base alloy studied here yields a family of glasses with a wide array of material properties. We now mostly know what this ‘microalloying’ does; we have hints of how it does it; we have little idea why. Beyond the fundamental physics of understanding such atomic interactions, this technique could generate materials with a host of applications, from bouncier golf clubs to stronger and lighter airplane wings (not to mention the imaging of infant brains!).
Have you ever noticed that the stages of human development are directly tied to our mastery of new materials? The Stone Age, Bronze Age and Iron Age define antiquity. In modern times, advances have been characterized by steel, polymers and silicon. In this talk I will give you an overview of these historical eras, and introduce you to current, cutting edge techniques that define the field of materials physics, the effort to produce, characterize and apply new materials from a fundamental framework. I will discuss our group’s work in basic physics of atomic structure of liquids and metallic glasses using the Beamline Electrostatic Levitator (BESL), and other techniques such as Electron Microscopy. If time permits I will discuss my work on hydrogen storage, and give some general perspective on renewable energy, including the lesson of Hero of Alexandria.
Finally, find out what actually goes on up on the locked fourth floor! The purpose of this talk is to provide an introduction to presolar grains and the reasons why we study them. Individual grains of dust from stars that lived and died before the Sun was born can be examined in the laboratory. These grains of stardust, called presolar grains, have been preserved in meteorites since the formation of the Solar System. Instead of using a telescope like most astrophysicists, we study these grains with microanalytical techniques like mass spectrometry (SIMS) and transmission electron microscopy (TEM). By studying the isotopic and chemical compositions, crystal structures, and morphologies of these grains, we can gain insight into stellar processes, such as nucleosynthesis, stellar mixing, galactic chemical evolution, and the physical conditions in stars.
From the perspective of an experimental physicist, three different missions to Mars will be discussed, together with their context. The Mars Exploration Rovers, 'Spirit' and 'Opportunity', have been exploring the surface of Mars for almost four years now. The visible/near-infrared CRISM imaging spectrometer has been acquiring hyperspectral images and it has been making multispectral maps of mineralogy on Mars from its bird's-eye view on the Mars Reconnaissance Orbiter for one year. And the Phoenix Lander has recently been launched, and should land safely in the northern latitudes of Mars in mid-2008 in order to dig for and analyze buried H2O ice.
The brain is the most complex system we know of. It is the result of an evolutionary process and consists of billions of interconnected neurons. Connectivity between neurons is neither random nor regular. We study signal processing with neural feedback loops using the avian isthmo-tectal loop as a model system. Isthmo-tectal is a precisely wired feedback loop in the avian visual system. We hope our work could contribute something to the answer of question addressed in the title: How does the chicken see the world?
Ultrasound has been widely used in diagnostic medicine for both imaging and tissue characterization. An introduction to the principles of tissue characterization is given, with a particular focus on using the cyclic variation of ultrasonic backscatter from myocardial tissue as an indicator of viability and performance. The origins of cyclic variation are discussed in terms of basic scattering principles and a three component Maxwell-type model. Motivations for ultrasonic mouse studies are also given. Data were taken on six wild type (normal) mice with relatively high frequency ultrasound. On all mice, cyclic variation measurements were performed and shown to be consistent with findings in humans and larger animals. These preliminary data demonstrate the feasibility of doing cyclic variation of integrated backscatter studies on mouse models with high frequency ultrasound.
So you may know that MRI is capable of giving us pretty images, but did you know that it can also give us information about structures much smaller than the best possible resolution of the image? Diffusion MRI allows us to measure surface to volume ratios of the alveoli in your lung (which have dimensions on the order of 100 μ m, smaller than the smallest resolution of conventional MRI, around 1 mm). Come find out how we’re using this information to study emphysema and come up with better and earlier ways to diagnose it.
Hydrogen gas and ice at P > 1 kbar and T ~ 250 K form H2-H2O clathrate in which four H2 and one H2 may occupy each large and small cage, respectively, of the sII hydrate structure. In H2-THF-H2O clathrate, THF singly occupy large cages whereas H2 occupies singly and only small cages. The novel hydrogen clathrate is an interesting compound from two aspects. One would see it as a prospective method of storing hydrogen. In addition, it provides a great opportunity to study molecular motions and interactions because it shows multiple cage occupancy. We performed 1H-NMR on both H2-D2O and H2-TDF-D2O clathrates to investigate dynamic properties, rotation and diffusion, of encaged H2 molecules. In other words, we want to know what kinds of molecular motions and interactions happen in the material. The NMR linewidth arises due to the intramolecular dipole interaction within H2 molecules. Our H2 NMR data, linewidth as a function of temperature, reflect random crystal fields from frozen cage-wall D2O orientations. Remarkably, the linewidth is proportional to 1/T over a wide range of temperatures from 12 to 120 K indicating temperature independent crystal fields. We find dramatic reductions in NMR linewidth of H2-D2O starting at 120 K demonstrating the onset of cage-to-cage diffusion. In case of H2-TDF-D2O, however, drastic line narrowing starting at 175 K indicates time-averaging of the crystal fields due to the fast reorientation of D2O molecules. Our observations are inconsistent with theoretical predictions as well as previous experimental studies. I will particularly discuss the results we published recently (Lasitha Senadheera and Mark Conradi, J. Phys. Chem. B 2007, 111, 12097-12102).
3-D atom probe tomography is an experimental technique that allows you to study the composition of any conductive material at almost atomic resolution. My research is focused on using this technique to probe the short and medium range order in aluminum based metallic glasses. I will discuss both the technique and the results that I have FINALLY achieved.
Complex metallic alloy systems have interesting properties that have many useful applications. However, the as-prepared structure of those alloys is important and often difficult to determine even using multiple techniques. This will be a talk about my work on the AlYFe Glass/Nanocystalline Alloy Composite. I'll frame the discussion in the context of our efforts to determine whether or not the as-quenched state of Al88Y7Fe5 prepared via the wheel quenching technique is truly amorphous or not and what the structural differences are. Data interpretation is tricky! Devitrification of this base glass is explored and I'll discuss some of the difficulties in understanding the transformation properties and processes that can take place.
As a material undergoes a phase transition from normal state to superconducting state it exhibits peculiar characteristics, which can be detected by various methods. But all these methods may not be accessible under pressure due to restrictions on sample size, typically less than 100 &mu m. In this talk I will briefly go through the methods used to search superconductivity under pressure with a special case for an inter metallic compound of CaLi2, which seems to have large number of electrons per unit cell as compared to MgB2. Does the structure similarity with MgB2 and the constituent elements being superconductors under pressure favor CaLi2 as a superconductor?
The brain is an incredibly complex system consisting of some 100 billion neurons that on average connect to ten thousand neighboring neurons. Each of these neurons communicates with each other via electrical signals contributing to a vast electrical storm that sweeps across the brain. Traditionally, many processes including the transmission of visual information have been considered as linear hierarchal processes from the eyes to the visual centers of the brain. Within the past ten years, this view has been challenged with research that indicates that feedback is critical to the processing of visual information. In this talk, I will present a delayed feedback model that employs finite difference equations and numerically generates the parameter phase space. The model demonstrates that small microcircuits of neurons are capable of complex dynamics with applications ranging from rhythm based cognitive maladies such as Epilepsy and Parkinsons to short term memory.
Each year billions of dollars are spent on performing various medical tests in the process of diagnosing disease. Whereas in physical systems knowing a few state variables will determine the state of the system COMPLETELY, it seems that due to the complexity of medicine, the amount of "state" variables needed for medical diagnosis has no bounds. Perhaps it is because physicians can bill for measuring more state variables, but I'd like to think that the reason has to do with the fact that physicists have yet to fully share the powerful "causality bag of tricks" with biologists.
Via fully general-relativistic numerical simulations of colliding neutron stars, it has recently been observed that critical phenomena occur at the threshold between the collapse of the merged object into a black hole and the formation of a hypermassive neutron star. This seminar focuses on the dynamics of this phenomenon and the implications it may have for realistic astrophysical systems of neutron stars.
Quantitative ultrasound has emerged as a viable tool for clinical bone quality evaluation. However, interactions between ultrasound and cancellous (spongy) bone are not yet fully understood. In particular, the phase velocities of ultrasonic waves propagating through cancellous bone are widely reported to unexpectedly decrease with frequency. This negative dispersion is in violation of the requirements of causality as imposed by the acoustic Kramers-Kronig relations that link attenuation and dispersion. Our Laboratory has proposed an explanation for the observed negative dispersion: that multiple wave modes interfere such that the resultant "mixed" waveform exhibits an apparent negative dispersion when analyzed conventionally under the assumption that only one mode is present. Such effects could therefore result in the measurement of "apparent" properties of cancellous bone, obscuring the underlying "true" properties as characterized by the individual interfering wave modes. Using simulations, we first show that multiple wave modes, each exhibiting a positive dispersion, can combine to form a waveform that exhibits an apparent negative dispersion. We then use simulated and experimentally acquired data to explore Bayesian probability theory as an approach for recovering the individual wave modes from data composed of interfering waves.
Spontaneous pattern formation is a ubiquitous phenomenon found throughout nature. The color scheme of butterfly wings, periodic ridges along sand dunes, the shape of a ram's horn, growth of bacteria colonies, and urban sprawl are just a few examples of self-arranging pattern formation mechanisms. Novel self-organization techniques of fabricating ordered nanostructures through pulsed laser-induced dewetting of thin metal films have shown potential as a possible nanomanufacturing solution to fulfill the promise of nanotechnology in a variety of areas: sensing, energy harvesting, and magnetic storage. Experiments indicate that laser melting of ultrathin metal films instigates two basic types of pattern formation mechanisms, “spinodal” dewetting and thermocapillary flow. The former, arising from a thin film hydrodynamic instability, yields an array of nanoparticles possessing a monomodal size distribution and short range spatial order in the nearest-neighbor particle spacings. Laser interference generates thermocapillary flow and leads to systems of nanostructures with long range spatial order. The type of pattern formation mechanism selected can be predicted purely on the basis of the timescales of the processes. The subsequent patterns are highly tunable, in which the size and spacing of the nanostructures are controlled by thermophysical material properties and experimental parameters.
Electrical transport in any device depends on both the intrinsic physical properties, i.e. the carrier concentration, interfacial barrier height, etc. and the extrinsic geometric properties, i.e. the shape of the device, the placement of the leads, the presence or absence of inhomogeneities, etc. Normally, transport studies focus on the physical properties and samples are designed to minimize the geometric contributions. However, we have shown that by careful design, the geometric contributions can be made dominant, which demonstrate a new class of EXX phenomena, where E = extraordinary and, to date, XX = magnetoresistance (MR), piezoconductance (PC), optoconductance (OC) and the newly developed electroconductance (EC).
In this talk, I will first give a brief introduction to EXX family and their working principles. Then I will focus on the EEC’s sample design and fabrication, modeling and experiment results. By a careful choice of sample geometry and material, we achieve a 5.2% EEC effect under 2.5kV/cm at room temperature. And the one to one correspondence between the sensor resistance and the applied electric field makes a new type of field sensor possible.
In recent years there has been enormous interest in developing a complete field theory which adequately unifies gravitation with the standard model. Many models which attempt to reconcile gravity and quantum mechanics have been proposed with each attempting to make new testable predictions. With this in mind, current experiments are on the brink of testing a large number of these scenarios. This talk is based on one of the tests our group is trying to do in a lab at WashU, making it an exciting and affordable alternative to the physics done at large particle colliders.
First, the motivations for doing these experiments will be laid out. Next our method is presented. Finally I will discuss a small sampling of the many things which must be considered when designing an experiment which attempts to reach the needed sensitivity.
Metallic, nanosized particles exhibit novel electromagnetic properties that have potential uses in a broad range of applications, including efficiency enhancement of photovoltaic devices and focused irradiation methods of cancer treatment. Fast pulsed-laser induced dewetting of nanoscopic metal films is a promising avenue to economically fabricate ordered nanoparticles and presents a platform to investigate multi-physics, nonlinear dynamical systems. Irradiating metal films < 30 nm with a uniformly intense laser beam induces pattern formation that exhibits short range spatial order and possesses a characteristic length scale that are tunable through experimental parameters. The interplay between the thin-film optics, thermal diffusion, and hydrodynamics all dictate the parameters that can be tuned and their effect on the final morphology and characteristic length scales of the dewetting system. Here, we present our modeling approaches to capture the essential physics of this pattern formation process and compare the model with experimental results.
When cosmic rays were first discovered in 1912, they were believed to be a new type of electromagnetic radiation. Since then, astrophysicists have determined that they really are very energetic, charged particles. In fact, the most energetic cosmic rays can reach energies of up to ~10^20 eV! In my talk, I will focus on galactic cosmic rays with nuclear charge 5 ≤ Z ≤ 28 (yes, they're elements on the periodic table!). I will cover their energy, composition, and spectra as seen by the Cosmic Ray Isotope Spectrometer (CRIS) on board the Advanced Composition Explorer (ACE), a fancy instrument that's been in space for the last 10 years.
I will introduce the topic of color superconductivity, the big brother of electrical superconductivity. I will highlight the similarities and differences between them. Then I will discuss the various different color superconducting phases and what makes them unique. Next I will turn to discussing neutron stars as a likely environment where we can find color superconductors. I will finish up by showing different results for phenomenological properties of these color superconducting phases in neutron stars.
Clinicians have been using the end diastolic pressure volume relationship to measure the left ventricular passive stiffness. However, physical principles tell us that this method is WRONG. We need to use other methods to measure the passive stiffness of the ventricle. In this talk, I will talk about basic cardiac physiology, the left ventricular stiffness measurement, the problem with the current method, and the solution to the problem. Since this is a physics talk, I will stress the physics principles behind the problem and the audience will be able to solve the problem before I give the answer. I will also talk about the validation of the answer to convince you that our conclusion is correct. The talk will be fun.
Everything you do, feel, or think has a neural cause. Every moment of your perceptions and choices and desires takes place within the firing of action potentials (spikes) within your brain. Yet there is no overarching theory of the brain. Feed forward networks have been studied in great detail, but comparatively little attention has been paid to feedback. Our group utilizes novel preparations to better examine the role of feedback in both brain slices and whole brain in vivo preps. I will explain some of the questions we are trying to answer, and show some data from the frog demonstrating some possible affects of feedback.
Dust grains condense in the stellar atmospheres of evolved stars and in supernova ejecta. These grains of stardust were then ejected into the interstellar medium and incorporated into the molecular cloud that collapsed to form the solar system. Extensive processing of molecular cloud material and stellar remnants in the solar nebula and on planetary bodies virtually wiped out most traces of their origin. However, some extraterrestrial materials were only minimally altered and preserve “presolar grains”.
Presolar silicate grains have been discovered in primitive meteorites in the NanoSIMS ion microprobe in the year 2004. Their small sizes (< 500 nm) make the mineralogical characterization of presolar silicate grains a non-trivial task. The Auger Nanoprobe with a spatial resolution of ~10 nm is an ideal tool to measure elemental compositions of the presolar silicate grains. My initial work involved calibrating the Auger Nanoprobe acquired in August 2006, for geological samples; this was followed by identifying O-anomalous grains in the NanoSIMS in Acfer 094 and characterizing them in the Auger Nanoprobe. Then I examined a subset of the presolar silicate grains which were 18O-rich in order to test if all the silicate stardust grains originate from a single supernova source.
Last Updated: 05/05/2008 Narelle Hillier