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Colloquium

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.

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Fall 2017

Thursday, August 24---Fall Faculty Assembly

Thursday, August 31

Alyson Brooks, Department of Physics & Astronomy, Rutgers University

Re-Examining the Astrophysical Constraints on the Dark Matter Model   (show abstract)

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

Thursday, September 7

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   (show abstract)

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

Thursday, September 14

Herbert Levine, Hasselmann Professor of Bioengineering and Director, Center for Theoretical Biological Physics, Rice University

A physicist looks at cancer metastasis   (show abstract)

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

Thursday, September 21

Raphael Pooser, Oak Ridge National Laboratory

Practical Quantum Sensing at Ultra Trace Levels with Squeezed States of Light   (show abstract)

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

Thursday, September 28

Rachael Beaton, Department of Astronomical Sciences, Princeton University

Engineering the Measurement of the Hubble Constant   (show abstract)

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

Thursday, October 5

Nicole Joseph, Department of Teaching & Learning, Peabody College, Vanderbilt University

The Complexities of Black Women and Girls in STEM   (show abstract)

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

Thursday, October 12---Fall Break

Tuesday, October 17, 3pm (NEW TIME), SC 4327 SPECIAL COLLOQUIUM

Zsuzsa Marka, Department of Physics, Columbia University, NYC

Multimessenger Astrophysics with Gravitational-Waves   (show abstract)

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

Thursday, October 19

David Hogg, Professor of Physics and Data Science, NYU

Making accurate physical measurements with data-driven models.   (show abstract)

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

Thursday, October 26---GUY & REBECCA FORMAN LECTURE

Mary James, A. A. Knowlton Professor of Physics and Dean for Institutional Diversity, Reed College

What Does Access Really Mean?   (show abstract)

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

Thursday, November 9---WENDELL HOLLADAY LECTURE

Ed Zganjar, Department of Physics & Astronomy, Louisiana State University

How electron spectroscopy can provide unique solutions to nuclear structure problems   (show abstract)

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

Thursday, November 16

Chris Stanton, Department of Physics, University of Florida

Coherent Phonons in Condensed Matter Systems   (show abstract)

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

Thursday, November 23---Thanksgiving

Thursday, November 30

David Hilton, Department of Physics, University of Alabama at Birmingham

Materials in Extreme Environments: Unlocking New Materials Physics in High Magnetic Fields    (show abstract)

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

Thursday, December 7

Patrick Young, School of Earth and Space Exploration, Arizona State University

Supernova Structure and Synthesis: Destruction and Creation in Massive Stars   (show abstract)

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

Spring 2018

Thursday, January 11

Vince Cinciolo, ORNL

Fundamental Physics at the Oak Ridge Spallation Neutron Source   (show abstract)

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

Thursday, January 18

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Thursday, January 25

Virginia Wheeler, Naval Research Laboratory

Host: R. Haglund

Thursday, February 1

Michael Strickland, Kent State

Quarkonium suppression in the quark-gluon plasma   (show abstract)

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

Thursday, February 8

Ekaterina Poutrina, Air Force Research Lab

Host: Sharon Weiss

Thursday, February 15

Jamie Nagle, University of Colorado, Boulder

Pushing out of our comfort zone for quark-gluon plasma formation    (show abstract)

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

Thursday, February 22

Gregor Neuert, Molecular Physiology and Biophysics, Vanderbilt University

Host: S. Hutson

Thursday, March 1---FRANCIS SLACK LECTURE

Gabriela González, LIGO and Department of Physics & Astronomy, Louisiana State University

Host: D. Ernst

Thursday, March 8---Spring Break

Thursday, March 15

Pierre Sikivie, University of Florida

Host: T. Weiler

Thursday, March 22

Ned Wingreen, Howard A. Prior Professor of the Life Sciences, Departments of Molecular Biology and Physics, Princeton University

Host: E. Rericha

Thursday, March 29

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Thursday, April 5

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Thursday, April 12

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Thursday, April 19

Margaret Turnbull, Global Science Institute

Host: N. Hinkel