Colloquia are held on Thursdays at 4pm in room 4327 (building 4) of the Stevenson Science Center unless otherwise noted. Click here for directions, or phone the department. A reception with the speaker is held at 3:40pm in Stevenson 6333.
---Fall Faculty Assembly
Alyson Brooks, Department of Physics & Astronomy, Rutgers University
Re-Examining the Astrophysical Constraints on the Dark Matter Model
The cosmological model based on cold dark matter (CDM) and dark energy has been hugely successful in describing the observed evolution and large scale structure of our Universe. However, at small scales (in the smallest galaxies and at the centers of larger galaxies), a number of observations seem to conflict with the predictions of CDM cosmology, leading to recent interest in alternative dark matter models. I will summarize a number of ways that including baryonic physics (the physics of gas and stars) can resolve the conflict between theory and observations, by significantly altering the structure and evolution of galaxies. Despite all of the successes of baryonic physics in reconciling CDM with observations, I will explain why alternative dark matter models are still viable and interesting.
Host: K. Holley-Bockelmann, F. Munshi
Jonathan McMurry, Associate Vice President for Research, Professor of Biochemistry, Kennesaw State University
Being Crash Davis Ain’t So Bad: Life in the Science Minor Leagues
In the 1988 baseball masterpiece, Bull Durham, Crash Davis is a career minor-league catcher tasked with preparing pitching prospect Ebby Calvin “Nuke” LaLoosh for the major leagues. Davis, an["/events/colloquium/colloquium-data-2017-2018.xml"] excellent older player who experienced only one brief call-up to “The Show,” is frustrated about being relegated to class A ball with the Durham Bulls in the twilight of his career. He ends the film setting the career minor league home run record before retiring. Seemingly every graduate student sets out to land a tenure-track job at a Research 1 university. But just like professional baseball players, the vast majority simply won’t make it to The Show. In this talk, a career ‘minor league’ scientist reviews his career path, critically reviewing the roles of talent, luck, ambition and choices have played. Discussion of academic career options beyond R1s will highlight possibilities for serious research programs at primarily undergraduate universities, the benefits of avoiding the R1 tenure grind and the fulfillment to be found in merely staying in the game. Practical advice to those considering applying for undergraduate institution jobs will precede QA. Attendees will be asked to ponder that while Nuke might make it to the majors, Crash gets the girl. And that (particularly when it’s Susan Sarandon) ain’t so bad.
Host: S. Hutson
Herbert Levine, Hasselmann Professor of Bioengineering and Director, Center for Theoretical Biological Physics, Rice University
A physicist looks at cancer metastasis
Most cancer deaths arise when then primary tumor metastasizes and cancer takes root in distant organs. From the point of view of cellular behavior, metastatic spread requires many capabilities (motility, immune resistance, avoidance of cell death due to lack of adhesion, and ability to grow in a foreign location) which seem beyond what is normally possible for cells in typical organs. To address this issue, we focus on the phenomenon of phenotypic plasticity, the idea that the nonlinear dynamics of cellular genetic networks can lead to transitions to states that are capable of these feats. These new phenotypes can be studied with the help of mathematical models both of the underlying network and of the resultant biophysical properties (such as motility). By revealing the factors most responsible for the formation of these aggressive cellular types, we hopefully can suggest new targeting therapies for what remains the most recalcitrant aspect of cancer.
Host: W. Holmes
Raphael Pooser, Oak Ridge National Laboratory
Practical Quantum Sensing at Ultra Trace Levels with Squeezed States of Light
Quantum sensors are devices that exploit quantum mechanical effects to obtain enhanced sensitivity over their classical counterparts. Sensors that exploit quantum noise reduction, or squeezed light, have seen renewed interest in recent years as a growing number of devices that utilize optical readout – from gravitational wave detection to ultra-trace plasmonic sensing at the nanoscale – have approached their absolute limits of detection as deﬁned by the Heisenberg uncertainty principle. At this limit, the noise is dominated by the quantum statistics of light (the shot noise limit when coherent light is used). Simultaneously, many devices, including nanoscale sensors, have reached tolerance thresholds in which power in the readout field can no longer be increased. Beyond these limits, squeezed light is required in order to further improve sensitivity in these platforms. In this colloquium we will give a brief overview of quantum optics and quantum noise reduction and place the concepts within the historical context of quantum sensing. Further, we will present our work geared towards producing practical, ubiquitous quantum sensors that break through the shot noise limit to achieve state of the art sensitivities beyond the capabilities of classical devices. We demonstrate atomic magnetometers, atomic force microscopes, compressive imaging, quantum plasmonic imaging, and ultra-trace quantum sensors with state of the art quantum noise levels well below the shot noise limit. We will outline how these devices enable quantum sensing in a ubiquitous, off-the-shelf configuration enhanced with squeezed light in order to beat the state of the art achieved in the analogous classical sensor for the first time
Host: R. Haglund
Rachael Beaton, Department of Astronomical Sciences, Princeton University
Engineering the Measurement of the Hubble Constant
The local expansion rate of the Universe, the Hubble constant, is one of the fundamental parameters in our current concordance cosmology and one that anchors the expansion history of the Universe. The resolution of the historical factor-of-two controversy in the Hubble constant nearly two decades ago (e.g., the Hubble Space Telescope Key Project; Freedman et al. 2001) has evolved into a 3.4-sigma tension between the traditional Cepheid-distance ladder measurements (Riess et al. 2016, Freedman et al. 2012, Freedman et al. 2001) and that determined from modelling anisotropies in the cosmic microwave background (CMB; Planck Collaboration et al. 2016). At the heart of the tension, is not only a difference in method, but also a fundamental difference in the state of the observed Universe: the distance ladder measures the local rate in the nearby universe (e.g., z~0), whereas the CMB anisotropy measurements uses the very young Universe (z ~1100). Resolution of the tension requires (i) a full scale evaluation of the systematic effects in either technique or (ii) “new physics” added to the standard cosmological model. The trigonometric parallaxes provided by Gaia in the near term permit an unprecedented opportunity to use alternative standard candles and construct a full end-to-end distance ladder without Cepheids. The Carnegie-Chicago Hubble Program is doing just that; we are in the middle of building a new distance ladder that relies on the tip of the red giant branch (TRGB; Beaton et al. 2016). As I will demonstrate, this not only provides a direct cross-check on the Cepheid path, but there are numerous advantages to using a distance indicator that, as a standard candle from old stellar populations, is nearly ubiquitously present low-crowding and low-extinction components of galaxies. More specifically, by being able to calibrate every ‘local’ SNe Ia and easily probing ever-larger volumes with JWST and WFIRST, the TRGB-based distance ladder paves a clear path to a 1% measurement within the foreseeable future.
Host: S. Stewart
Nicole Joseph, Department of Teaching & Learning, Peabody College, Vanderbilt University
The Complexities of Black Women and Girls in STEM
The experiences of Black women and girls in STEM, mathematics in particular, is an understudied line of inquiry. We know very little about how they experience mathematics teaching and learning across the pipeline. The aim of this interactive talk is to problematize and interrogate the current circumstances surrounding Black women and girls in mathematics that deny them access, power, participation, and opportunity to develop mathematics identities.
Host: S. Hutson
Zsuzsa Marka, Department of Physics, Columbia University, NYC
Multimessenger Astrophysics with Gravitational-Waves---SC 4327 SPECIAL COLLOQUIUM
This year's Nobel prize in physics was awarded for the observation of gravitational waves with the LIGO detectors. Advanced LIGO's second observing run ended on August 25, 2017. Gravitational-wave data was analyzed near-real time to rapidly enable comprehensive multimessenger analyses, searching for the electromagnetic and neutrino counterparts. The history and status of the global multimessenger effort will be discussed.
Hosts: S. Hutson, N. Tolk
David Hogg, Professor of Physics and Data Science, NYU
Making accurate physical measurements with data-driven models.
We have spent more than a century building elaborate (computational) physical models, some of which are extremely successful. These include (for example) QCD, cosmological structure formation, and Earth climate. Despite their successes, these physical models disagree with the data in structured and repeatable ways, and we have enormous amounts of data to discover, test, and measure these disagreements. In many cases, we can combine good physical knowledge with good data to build a predictive model that is more powerful than any model built with either used on its own. Here I demonstrate these general points with the specific example of red-giant stars, where our data-driven model is now delivering more precise measurements of detailed stellar chemical compositions (the products of stellar nucleosynthesis) than any purely physical model. I will give comments on statistics and criticisms of machine learning that are relevant to many scientific domains.
Host: J. Bird
Mary James, A. A. Knowlton Professor of Physics and Dean for Institutional Diversity, Reed College
What Does Access Really Mean?---GUY & REBECCA FORMAN LECTURE
Access to education for prospective physics majors is more than permission to walk though the doors or sit in a classroom. Providing true access means creating a learning environment that anticipates the challenges to learning that students from different backgrounds and identities, particularly those from groups underrepresented in physics, bring to the classroom. In this talk Professor James will present findings from neuroscience, cognitive psychology, social psychology, and physics education research that suggest a new paradigm for physics courses and advising of prospective physics majors focused on increasing the persistence of students from all backgrounds and identities interested in the discipline.
Host: D. Ernst
Ed Zganjar, Department of Physics & Astronomy, Louisiana State University
How electron spectroscopy can provide unique solutions to nuclear structure problems---WENDELL HOLLADAY LECTURE
The presentation will introduce the basic physics behind the nuclear decays that take place by internal conversion and internal pair formation. Based upon highly successful theoretical descriptions of atomic electrons and the manner in which they interact with the nuclear core, a precise theory has been achieved that connects energy, spin, and parity changes in nuclear transitions to the emission of electrons (from atomic orbitals) as mediators of nuclear de-excitation. The same can be said for internal pair formation which supplements the internal conversion process in that the pair formation decay rate is largest where the internal conversion rate is smallest. A large body of nuclear structure data has been built on this less well known spectroscopy. More recent developments show that changes in nuclear shape, for example, can be revealed by such spectroscopy through the identification and quantification of electric monopole transitions. A brief discussion of specialized instrumentation for such measurements will also be presented. Finally, several examples of how precise knowledge of the structure of certain nuclei is important to astrophysics and studies of fundamental symmetries.
Host: J. Hamilton, A. Ramayya
Chris Stanton, Department of Physics, University of Florida
Coherent Phonons in Condensed Matter Systems
Ultrafast laser spectroscopy has proven to be a powerful technique for studying dynamical processes in semiconductor nanostructures. Since femtosecond experiments probe on the same time scale as scattering, they provide detailed information beyond standard D.C. mobility measurements on electron and hole carrier dynamics. In addition, the ultrafast optical photoexcitation of hot electrons and holes in semiconductors can trigger coherent phonon oscillations that can be probed in time-resolved differential reflection or transmission measurements. We will discuss the microscopic generation and detection of coherent phonons in a variety of condensed matter systems ranging from bulk semiconductors to heterostructures/ superlattices to carbon nanotubes. We then show how both coherent optic an acoustic phonons can be used to all-optically probe and characterize the internal electric fields, energy bands, carrier distribution functions and many-body interactions at the surface as well as at buried interfaces. Finally, we demonstrate that materials can be tailored to enhance the generation or detection of coherent phonons creating the possibility of novel devices based on phononic properties.
Host: N. Tolk
David Hilton, Department of Physics, University of Alabama at Birmingham
Materials in Extreme Environments: Unlocking New Materials Physics in High Magnetic Fields
We are constantly pushing materials into new regimes and extremes to try to understand how they function. How fast can the electronic or optical properties of a material be modulated? How do they operate under thermodynamic extremes of temperature, pressure, and/or magnetic field? As we push these materials to these new extremes, are we elucidating new physics or can they be explained using extensions to conventional descriptions of their properties? Novel two-dimensional materials are one promising platform for next-generation devices that push the limits of both speed and size, but they also require new descriptions and experimental tools to describe their novel properties. In these newer two-dimensional materials like graphene and transition metal dichalcogenides, the relatively short coherence times of these still-developing materials masks some of their unique capabilities for next generation novel electronics. The modulation doped gallium arsenide two-dimensional electron gas (2DEG), in contrast, has seen and continues to see extensive study as one of the more “traditional” platforms for 2D materials. High quality samples with mobilities exceeding >106 cm2 V-1 s-1 are currently available, which provides a model system to study the electronic and optical properties of two-dimensional materials in the “clean” limit. Traditional measurement in these materials have included a variety of electrical transport measurements [e.g. Phys. Rev. Lett. 48, 1559 (1982)] and time-integrated optical measurements [e.g. Phys. Rev. B 31, 5253 (1985)], while the study of their dynamic properties on subpicosecond time-scales is relatively recent [e.g. Phys. Rev. B 93, 155437 (2016)]. Ultrafast spectroscopic techniques are a powerful technique that can be used to unravel complex and often competing processes in condensed matter systems on a femtosecond time scale. High magnetic field spectroscopy is also a particularly useful optical tool for unraveling complex interactions in these systems, which are a particularly rich source of novel materials physics due to the relative absence of disorder in two-dimensional electron gases. In this talk, I will discuss our work using terahertz time-domain spectroscopy to study Landau level populations and coherences in high mobility two-dimensional semiconducting systems and our extensions of these techniques to higher magnetic field spectroscopy. We model our results using the Optical Bloch Equations to determine the dephasing lifetime as a function of temperature and explain our low temperature results using ionized impurity and bound interface charge scattering in the conducting layer. In the second part of my talk, I will discuss our recent work to study these materials in high magnetic field using the 25 Tesla Split-Florida Helix at the National High Magnetic Field Lab. Our results reveal a complex interplay between conventional (electron transport) and complex (many-body) electronic interaction on an extremely fast time scale. These results have their origin in the breakdown of the frequency used uniform electron gas description of conductivity in high quality two-dimensional electron gas systems that happens when the magnetic length is on the same order as the material’s lattice constants.
Host: K. Hallman, R. Haglund
Patrick Young, School of Earth and Space Exploration, Arizona State University
Supernova Structure and Synthesis: Destruction and Creation in Massive Stars
Chemical composition plays a fundamental role in the evolution of stars and even more so of planets. Lines of evidence ranging from the isotopic abundances in meteorites to large surveys of stellar elemental abundances make it clear that the detailed composition of star systems varies significantly and can even be affected during their formation. The explosion of short-lived massive stars as supernovae synthesizes and delivers new elements to the surrounding interstellar gas, potentially enriching nearby protostellar and protoplanetary material. I will present results of 3D simulations of supernova explosions followed into the supernova remnant phase and comparisons to observations of supernova remnants. By examining the formation and development of structures in the explosion and their subsequent interaction with circumstellar material we gain insight into the supernova mechanism itself and how the diversity of chemical composition in stellar systems is produced.
Host: N. Hinkel
Vince Cianciolo, ORNL
Fundamental Physics at the Oak Ridge Spallation Neutron Source
The Spallation Neutron Source (SNS) at Oak Ridge National Lab is focused on materials science. However, there is a small but robust fundamental (i.e., nuclear/particle) physics program as well. Measurements of the hadronic weak interaction in simple systems (polarized neutron capture on hydrogen and helium-3 targets) have recently been completed. Precision measurements of angular correlations in neutron beta decay will commence soon, allowing tests of CKM unitarity. Significant advances have been made in the development of an experiment to significantly improve the measurement of the neutron’s electric dipole moment in a quest to understand the reason for the existence of matter in the universe. A neutrino research effort, currently focused on detection of coherent neutrino/nucleus scattering, has also started. This talk will introduce each of these elements of the SNS nuclear/particle physics program.
Host: J. Velkovska
Kai Xiao, Center for Nanophase Materials Sciences,Oak Ridge National Laboratory
Heterogeneity in 2D materials: From localized defects to macroscopic heterostructures
Two-dimensional (2D) materials are intrinsically heterogeneous, therefore controlling defects, understanding the impact of boundaries and interfaces, and developing means to exploit these heterogeneities is a transformative opportunity that could underpin future technologies and energy applications. In this talk, I will discuss the fundamental understanding of the roles of heterogeneities including defects, dopants, edges, strain, phases, and atomic interface in 2D materials and their heterostructures. Through isoelectronic doping in monolayer of MoSe2, the Se vacancies are effectively suppressed and photoluminescence is significantly enhanced due to the decrease of defect-mediated non-radiative recombination. In addition, we demonstrate the non-equilibrium, bottom-up synthesis of single crystalline monolayers of 2D MoSe2−x with controllable levels of Se vacancies far beyond intrinsic levels. Both substitutional dopants and vacancies were shown to significantly alter the carrier properties and transport characteristics within a single monolayer (e.g., n- to p-type conduction in W-doped MoSe2 and in Se-deficient MoSe2-x). The vertical and lateral 2D heterostructures by controlled assembly and doping will be discussed. The bottom up synthesis of 2D materials discussed here provides excellent control over the heterogeneity in 2D materials, which can tunably modulate the optical and electrical properties in 2D materials and their heterostructures for ultra-thin and flexible solar-energy devices and nano-electronics.
Host: R. Haglund
Virginia Wheeler, U.S. Naval Research Laboratory
The Power of Atomic Layer Deposition – Moving Beyond Amorphous Films
Atomic layer deposition (ALD) has emerged as a powerful technique to produce a wide variety of thin film materials including oxides, nitrides, and metals for use in numerous applications. This method has become increasingly useful as device dimensions are reduced and non-planar complexity is increased. The sequential, self-limiting reactions that define ALD enable excellent conformality on high-aspect ratio structures, angstrom level thickness control, and tunable film compositions. Additionally, ALD is conducted at low growth temperatures (Tg) which allows for integration of dissimilar materials or soft materials as well as the ability to access new regions of phase diagrams in complex systems (i.e. metastable phases, miscibility gaps, etc). While the low Tg of ALD traditionally yields amorphous films, many emerging applications would benefit from an ability to incorporate thin, conformal crystalline materials. To extend crystalline capability to ALD, many have investigated post-deposition processing or plasma enhanced ALD. In this work, we will explore the advantages and limitations of approaches towards attaining crystalline ALD films through the following case studies: high quality phase transitions in ALD VO2, achieving full stoichiometric range of ternaries in crystalline III-N films, and phase control of heteroepitaxial Ga2O3 and TiO2.
Host: R. Haglund
Michael Strickland, Kent State
Quarkonium suppression in the quark-gluon plasma
At a temperature of approximately T_QGP ~ 155 MeV (~ 1.5 x 10^12 K) quantum chromodynamics predicts that nuclear matter undergoes a phase transition to a deconfined and chiral-symmetry restored phase called the quark-gluon plasma (QGP). As the temperature of matter increases to and then above T_QGP one expects the sequential melting of quark bound states, ordered by the masses/binding energy of the various states. As a result, light hadronic states, such as the pion dissociate around T ~ T_QGP, however, heavy quark bound states can survive in the QGP to much higher temperatures, e.g. the ground state of a bottom and anti-bottom quark, the Y(1s) particle, survives in the QGP up to temperatures on the order of 600 MeV. As a result, the production of heavy-quark bound states in the heavy-ion collisions relative to their production in proton-proton collisions at the same nucleon-nucleon center-of-mass energy can be used to establish the formation of the QGP and to infer its properties e.g. the initial temperature of the QGP, its shear viscosity, etc. In this colloquium, I will review the basics of quarkonium suppression and present experimental evidence for this "smoking gun" for the creation of the QGP in ultra-relativistic heavy-ion collisions.
Host: J. Velkovska
Jamie Nagle, University of Colorado, Boulder
Pushing out of our comfort zone for quark-gluon plasma formation
The quark-gluon plasma is a high temperature (> a trillion Kelvin) state of matter where quarks and gluons are no longer confined inside hadrons. This plasma is studied in the laboratory via nuclear collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider, and displays some remarkable properties including near perfect fluidity. Recent experiments have revealed similar fluidity signatures in collisions of small systems, including proton-proton and proton/deuteron/helium-nucleus reactions. These challenge our understanding of the requirements necessary for plasma formation and the applicability of hydrodynamic modeling.
Host: J. Velkovska
Gregor Neuert, Molecular Physiology and Biophysics, Vanderbilt University
Predictive understanding of cell biological systems through kinetic analysis
Despite robust efforts over the years, it has proven very difficult to identify mathematical models that would improve biological insight by predicting complex biological responses, as needed to accelerate the design of medical treatments. This problem remains unsolved because large-scale models, with hundreds of unknown reaction rates, may be too complex to be supported by existing experimental techniques or data sets, and therefore may provide little quantitative insight. At the other extreme, overly simple models ignore the intricacies of real biological processes and may equally be limited in their ability to predict real phenomena. Models are often further limited by the fact that most experimental analyses only probe average equilibrium characteristics of cell populations and ignore potentially useful information contained in measurable fluctuations in space, time and environment, and from one cell to another. This is a fundamental problem in all of biology, because models and parameters that are identified from measurements of population of cells do not capture the variability in biological processes and therefore these models are potentially misleading. In essence, models inferred from population averages can fit the population data very well but they may not predict very well. One key to overcoming these limitations is to generate single-cell and single-molecule experimental data sets of high quality and reproducibility that capture the variability in biological processes. Because single-cell data contain information hidden in population averages, I will demonstrate how cellular systems identification methodology of integrating quantitative single-cell experiments with stochastic mathematical models is maximally predictive. Our approach is general and may be applied to any measurement that detects variability and any biological process that exhibits variability.
Host: S. Hutson
Gabriela González, LIGO and Department of Physics & Astronomy, Louisiana State University
Gravitational Waves Astronomy---FRANCIS SLACK LECTURE
The discoveries of gravitational waves from mergers of black holes and neutron stars have opened a new era of gravitational wave astrophysics, with very bright prospects for the future. We will describe the details of the latest discoveries of mergers of binary black hole systems, the observation of a merger of neutron stars by LIGO and Virgo detectors that was followed up by many electromagnetic observations, and the exciting prospects for more detections in the next years.
Host: D. Ernst
Pierre Sikivie, University of Florida
The Search for Axions
The axion is a hypothetical particle proposed, approximately 40 years ago,to explain why the strong interactions are P and CP invariant. Additional motivation for its existence comes from the fact that a cold population of axions is naturally produced in the early universe. These cold axions may constitute the dark matter today. I'll briefly review the limits on the axion from particle physics, stellar evolution and cosmology. The various constraints suggest that the axion mass is in the micro-eV to milli-eV range. In this range, its interactions are so weak that the axion was once thought invisible. Nonetheless a number of methods have been proposed to search for so-called "invisible" axions. I'll describe these techniques, the experiments that have implemented them, and the results that have been obtained so far.
Host: T. Weiler
Ned Wingreen, Howard A. Prior Professor of the Life Sciences, Departments of Molecular Biology and Physics, Princeton University
Host: E. Rericha
Jenny Greene, Professor of Astrophysical Sciences, Princeton University
Going wide and deep with the Hypersuprime Camera Survey
Our ongoing imaging survey with the Hypersuprime Camera (HSC) on the Subaru Telescope provides a rich data set for studying galaxy evolution, from the most massive elliptical galaxies to ultra low surface-brightness dwarfs. I will discuss ongoing projects that utilize the deep and wide HSC imaging to address the role of merging in the growth of black holes and galaxies, as well as our search for some of the most extreme low surface-brightness galaxies.
Host: K. Stassun
Ekaterina Poutrina, Air Force Research Lab
Host: Sharon Weiss
Yohannes Abate, Department of Physics, University of Georgia
Nano-Spectroscopic Probing of Materials
Host: R. Haglund
Margaret Turnbull, Global Science Institute
Where Things Stand with the First Exoplanet Direct Imaging Flight Mission
Among other astrophysical “firsts,” the WFIRST mission will be the first demonstration of exoplanet coronagraphy in space. The coronagraph instrument (CGI) is intended to demonstrate several key technologies that are on the critical path to larger missions that will find and spectrally characterize planets that could be habitable to life as we know it. WFIRST entered Phase A in January of 2016, and is expected to enter Phase B in April of this year. This talk will describe the story of how this mission came to be, where things currently stand in terms of predicted imaging performance, the potential for a starshade rendezvous mission, and what to expect for guest observer opportunities. I’ll also describe how the two coronagraph science teams are working to maximize the scientific output of what is categorized as a “technology demonstration” instrument. Finally, I'll offer some of my personal take-aways from the experience of watching such a large and challenging mission come together.
Host: N. Hinkel