George Mason University Department of Physics and Astronomy Colloquium Schedule 2018-2019
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Day of Week
TimeLocationSpeakerOrganizationTitle / TopicAbstractBiographyScheduleNotesHost
First week of semester - No speaker
9/7/2018FridayNo speaker
9/11/2018Tuesday3:15 PMEXPL 3301Rajiv SinghUC DavisTitle: The quest for Quantum Spin Liquid phases of matterQuantum Spin Liquids represent highly quantum-entangled phases of matter that support particles with fractional attributes of the electron and emergent gauge-fields. The advent of simple soluble models, techniques from quantum field theory and a quiet revolution in computational methods, combined with synthesis of new materials and extraordinary advances in cutting-edge experimental techniques have brought us to the brink of the discovery of Quantum Spin Liquid phases of matter. We will discuss recent theoretical developments, different material candidates for Quantum Spin Liquids and the growing body of evidence for their existence.
Special Date and time
Predrag Nikolic
9/14/2018Friday3:00 PMEXPL L003Vivienne BaldassareYaleSearching for Active Galactic Nuclei in Low-Mass GalaxiesThe population of massive black holes at the centers of nearby low-mass galaxies provides some of the best observational constraints on the masses of “black hole seeds” at high redshift. Furthermore, while massive black holes are ubiquitous and well-studied in Milky Way-sized and larger galaxies, relatively little is known about the population or properties of those in smaller galaxies. I will discuss recent observational efforts to find accreting massive black holes in low-mass galaxies via an array of multi-wavelength techniques, concentrating on searches using optical spectroscopy and optical photometric variability. I will also describe the multi-wavelength properties of active galactic nuclei in low-mass galaxies and discuss how they compare to more massive systems. Finally, I will discuss the properties of the active dwarf galaxy RGG 118, host to the smallest reported nuclear black hole.Einstein Fellow Satyapal
9/21/2018Friday3:00 PMEXPL L003Alex WiseU DelawareNew Methods for Finding Activity-Sensitive Spectral Lines: Combined Visual Identification and an Automated Pipeline Find a Set of 40 Activity Indicators
Stellar activity causes radial velocity variations that can either mimic planets or hide their existence. To verify the authenticity of newly discovered planets, observers may search for periodicity in spectral lines such as Ca H & K and H ⍺, then mask out any radial velocity signals that match the activity period or its harmonics. However, not every spectrograph includes Ca H & K, and redder activity indicators are needed for planet searches around low-mass stars. Here we show how new activity indicators can be identified by correlating spectral line depths with a well-known activity index. We apply our correlation methods to archival HARPS spectra of ε Eri and ⍺ Cen B and use the results from both stars to generate a master list of activity-sensitive lines in the visible spectrum. Our newly discovered activity indicators can in turn be used as benchmarks to extend the list of known activity-sensitive lines toward the infrared or UV. With recent improvements in spectrograph technology, stellar activity is now the biggest noise source in planet searches. Our suite of > 40 activity-sensitive lines is a first step toward allowing planet hunters to access all of the information on stellar activity contained in each spectrum.
Alex Wise earned his B.A. in physics from the State University of New York at Geneseo in 2012. He is now finishing up his Ph.D. in physics at the University of Delaware. Alex is the 2017 and 2018 recipient of the Delaware Space Grant Fellowship. Alex has previously researched giant planet formation, and currently studies precise stellar spectra for understanding stellar activity as it impacts the radial velocity method for the detection of Earth-mass exoplanets.
Peter Plavchan
9/28/2018Friday3:00 PMEXPL L003John V. Shebalin
George Mason
The Origin of the Earth’s Magnetic Dipole Field
The geological record tells us that the Earth has long had a quasi-stationary magnetic dipole field (mostly axial and subject to reversals), along with less energetic higher-order multipole components. The geomagnetic field has been very important for the development of life on Earth because it provides a shield against space weather. It is now generally accepted that this global magnetic field arises from dynamical processes in the Earth’s electrically conducting, molten iron, turbulent fluid core. Understanding the origin of the geomagnetic field depends on observational data concerning the interior structure of the Earth, as well as on computational and theoretical discoveries in the physics of turbulent, electrically conducting fluids. Here, we review the pertinent history, geology and physics of the subject, including new results as to why the Earth’s magnetic dipole field tends to line up with its rotation axis, as well as a possible mechanism for polarity reversal.
Peter Plavchan
10/5/2018Friday3:00 PMEXPL L003Ramamurti Shankar YaleA crash course in relativityThis talk is a gentle introduction to some of the key
ideas in relativity. It will provide a brief history of past discoveries that set the stage for Einstein's work and then his main ideas. It is aimed at the general public and all that is required is an open mind. No child left behind!
Professor R. Shankar is John Randolph Huffman Professor of Physics at Yale University. His research is in theoretical condensed matter and particle physics. He is a Fellow of the American Academy of Arts and Science and was the recipient of the 2009 Julius Edgar Lilenfeld Prize of the American Physical Society. Professor Shankar is also known for his highly-regarded textbooks, including Principles of Quantum Mechanics, Fundamentals of Physics, and Quantum Field Theory and Condensed Matter: An Introduction.
Branislav Djordjevic, Indu Satija
10/12/2018Friday3:00 PMEXPL L003Sugata ChowdhuryNISTAnalysis and novel understanding of Raman active modes in CDW phase of Ta-dichalcogenides using DFT

Metallic transitions metal dichalcogenides (TMDCs), such as tantalum dichalcogenides (TaX2 (X=Se,S)), display interesting properties such as superconductivity or charge density wave (CDW) states at low-temperatures. In this work, we investigate the CDW transitions in bulk 2H-TaX2 using density functional theory (DFT) and discussed the origins to all CDW modes including previously unanalyzed modes. We found that, the natures and directions of atomic vibrations of all those new Raman modes are different and depend on temperatures. Our DFT calculations revealed that the atoms mostly vibrate in a circle in commensurate CDW and in a straight line in incommensurates CDW (ICCDW) phase. As an example, high-frequency E_2g^"1" mode of 2H-TaSe2, is very sensitive to the distortion of the atomic position as the CDW phase forms: at room temperature all the atoms vibrate in the direction of the atomic rows, but around 100 K (ICCDW) the direction of the vibrations rotates by a 30º angle. We also show the evolution of the lattice structure as a function of temperature and identify the equilibrium structure of the CDW phase, showing that it depends on the material (stripes versus triangles). Our results shed a light into the relationship between Raman modes and atomic displacement in the CDW state of other layered materials.
Patrick Vora
10/19/2018Friday3:00 PMEXPL L003Hsun-Jen ChuangNRLIndirect transition and opposite circular polarization of Interlayer Exciton in a MoSe2/WSe2 van der Waals Heterostructure Van der Waals heterostructures (vdWHs) is an emergent new class of heterostructures where monolayer semiconductors such as transition metal dichalcogenides (TMDs) are mechanically transferred and stacked in any arbitrary order. One unique property that arise from such heterostructures is an interlayer exciton (ILE), a spatially indirect electron-hole pair with the electron in one TMD layer and the hole in the other. However, the observation of such ILE had been rare, likely due to the contaminants trapped between layers during the transfer process. Here, using a clean dry transfer method as well as a new “nano-squeegee” technique utilizing an atomic force microscopy (AFM) tip, we create MoSe2/WSe2 heterostructures encapsulated in hBN with clean interfaces. We observe ILE emission around 1.35 eV at room temperature which can be clearly resolved into two distinct peaks (ILE1 and ILE2) separated by 24 meV at zero field at 5 K. Interestingly, these two emissions also exhibit opposite circular polarizations: up to +20% for ILE1 and -40% for ILE2 when excited with circularly polarized light. Ab initio calculations indicate that this is a result of the doubly indirect character of both electronic transitions: they are indirect in both real and reciprocal space, contrary to previous belief, split by relativistic effects.
Dr. Hsun-Jen Chuang is currently an ASEE postdoctoral fellow at the Naval Research Laboratory in Washington D.C., working with Dr. Berry Jonker in the Materials Science and Technology Division. Dr. Chuang received his Ph.D. in Physics from Wayne State University in 2016, after a M.Sc. from Western Illinois University in 2011 and B.Sc. from Chinese Culture University in 2006 in Taiwan. His research focuses on exploring the novel optical and electrical properties of layered two-dimensional semiconducting materials such as transition metal dichalcogenides (TMDs) and their heterostructures for the electronics applications, where he has developed clean dry transfer methods, as well as a “nano-squeegee” technique that utilizes an atomic force microscope tip to create clean interface. Dr. Chuang has published 15 papers, and received the Best Thesis Award at Wayne State in 2016, Best Postdoctoral Poster Award at the Naval Research Laboratory in 2017, and several poster awards at scientific conferences. Vora
10/26/2018Friday3:00 PMEXPL L003Utpal ChatterjeeUVa
Normal state of High Temperature Superconductors

Even though High Temperature Superconductors (HTSCs) were discovered three decades ago, a microscopic theory is yet to be realized for this unique class of materials. An important step towards this is to characterize the normal state of the HTSCs in great detail. From our temperature (T) and carrier concentration (d) dependent Angle Resolved Photoemission Spectroscopy (ARPES) measurements on Bi2Sr2CaCu2O8+δ (BISCO 2212) HTSCs, we find that unlike in conventional superconductors where there is a single temperature scale Tc separating the normal from the superconducting state, HTSCs are associated with two additional temperature scales. One is the so-called pseudogap scale T*, below which electronic excitations exhibit an energy gap. The second is the coherence scale Tcoh, below which lifetimes of quasiparticles get enhanced. We observe that T*( d) and Tcoh (d) cross each other near optimal carrier concentration, i.e. the d for which Tc (d) attains its maximum value. There is an unusual phase in the normal state where the electronic excitations are gapped as well as coherent. Quite remarkably, this is the phase from which the superconductivity with maximum Tc emerges. We also conduct direct comparison between the single-particle spectral functions from charge density wave systems and from the pseudogap phase of BISCO 2212 HTSCs. Our data do not seem to be consistent with the propositions that the energy gap for T< T* is due to some charge ordering. Rather, our data are consistent with the presence of incoherent pairs in the pseudogap phase. Moreover, our experimental finding that the two crossover lines T*(d) and Tcoh (d) intersect is not compatible with the theories invoking “single quantum critical” point near optimal doping, rather it is more naturally consistent with theories of superconductivity for doped Mott insulators.

Dr. Utpal Chatterjee is an assistant professor at the physics department of the University of Virginia. His research area is experimental condensed matter physics with an emphasis on the study of electrical and electronic properties of complex materials, which host exotic quantum states due to collective dynamics of a very large number of electrons. Some of these materials are cuprate high temperature superconductors, colossal magneto resistive manganites, transition metal dichalcogenides, and narrow band gap semiconductors. To explore various physical phenomena in these systems, he employs an experimental technique known as angle resolved photoemission spectroscopy (ARPES). Apart from using the ARPES equipment at his lab, he also travels to the user facilities worldwide for synchrotron-based ARPES experiments. Dr. Chatterjee completed his Ph.D in physics from the University of Illinois at Chicago (UIC) in 2007. He stayed at UIC for another two years as a postdoctoral research associate. In 2009, he moved to the Materials Science Division of the Argonne National Laboratory for his postdoctoral work, where he spent three years before joining the University of Virginia in 2012.
Patrick Vora
11/2/2018Friday3:00 PMEXPL L003Bradford BehrTornado Spectral SystemsHigh-resolution spectroscopy near and far (10^-3 to 10^20 meters)Optical spectroscopy is a tremendously powerful method for measuring the physics and chemistry of "distant" objects. In some cases, these objects might be ancient stars at the edge of our Galaxy. In other cases, the spectroscopic target may be a pharmaceutical mixture in a small glass vial in the lab. Regardless of the distance of the objects of interest, many of the same spectroscopic principles apply, and optical hardware originally developed for one type ofresearch can sometimes be fruitfully applied to other domains. I will describe one such development trajectory, following a specific spectrometer concept as it evolved from academia to government to industry over a span of ~20 years, performing a lot of interesting science along the way.
Peter Plavchan
11/9/2018Friday3:00 PMEXPL L003Joey RodriguezHarvardPlanetary Archaeology: Understanding Planet Formation from Eclipsing Disks and Compact Planetary ArchitecturesThe success of ground-based transit and RV surveys, and the Kepler/K2 mission, has shifted the exoplanet field from pure discovery to a combination of discovery, demographic analysis, and detailed characterization, especially for exoplanet atmospheres. Unfortunately, most known transiting exoplanet hosts are too faint to permit atmospheric characterization. We are using data from the TESS, K2, and ground-based transit surveys like the Kilodegree Extremely Little Telescope (KELT) project to find planets around bright stars while addressing specific questions about planet formation and evolution. We are also studying the birthplaces of planets by searching for occultations of newly formed stars by their protoplanetary disks with our Disk Eclipse Search with KELT (DESK) survey. These systems provide insight into the conditions required for planet formation. I will describe our results and discuss how we will search for these kinds of objects in future surveys such as LSST.
Joey Rodriguez is a Future Faculty Leaders Postdoctoral Fellow at the Harvard-Smithsonian Center for Astrophysics. He received his Ph.D. from Vanderbilt University for using transiting exoplanets and eclipsing disks to understand planet formation and evolution. Currently, his focus is discovering new unique exoplanetary systems that provide insight into key questions about planet formation using observations from NASA’s Kepler, K2, and TESS missions, and ground based transit surveys like KELT and MEarth. Plavchan
11/16/2018Friday3:00 PMEXPL L003Michelle Thaller
NASA Goddard
NASA Science Now: Current Events and Upcoming Missions
With over 100 active science missions, it’s a challenge to keep up with all the news coming out of NASA. Just in our solar system, we’ve got a lot going on in the coming months. Our next Mars landing is November 26, as the InSight Mission touches down to study the deep interior of the Red Planet. We’re also approaching the asteroid Bennu, with the goal of returning a sample of pristine material from the ancient solar system in 2021. And this January, we fly by a distant Kuiper Belt Object, visiting a part of the solar system that has never been seen before. Add on a new exoplanet-finding mission, a search for the source of the acceleration of the universe, and a giant space laser measuring changes in the polar ice, and you have some idea of the breadth and excitement of the science going on at NASA right now.
Dr. Michelle Thaller is the Assistant Director of Science at NASA's Goddard Space Flight Center. Michelle has a Bachelor’s in astrophysics from Harvard and a Ph.D. from Georgia State University. After a post-doctoral research fellowship at Caltech, Michelle became particularly interested in public outreach and science
communication and served as the public outreach lead for the Spitzer Space Telescope at NASA’s Jet Propulsion Laboratory before moving to Goddard in 2009,
where she currently serves as the Assistant Director of Science. Michelle has also worked at NASA Headquarters in Washington, D.C. on strategic communications. 
Outside of her work at NASA, Michelle is one of the regular hosts of Discovery Science Channel’s “How the Universe Works” and "Space's Deepest Secrets." Michelle also hosts the podcast “Orbital Path” on public radio.  She has received several high-profile awards for online science journalism and science leadership. Plavchan
Thanksgiving break - No talk
11/30/2018Friday3:00 PMEXPL L003Knicole Colón
NASA Goddard
Characterizing K2 and TESS Exoplanets with Near-Infrared, Ground-Based Transit PhotometryI will present results from an ongoing program that uses the 3.5-meter WIYN telescope to obtain high-precision, high-cadence, high-spatial-resolution near-infrared transit photometry of exoplanets and candidates discovered by NASA’s K2 mission. These observations support the confirmation and characterization of these planets, many of which are sub-Neptune-size and orbit bright, nearby, cool stars. With WIYN, we refine the planet sizes thanks to minimization of stellar limb darkening in the near-infrared. We also provide new measurements of the transit ephemerides, which is necessary due to the relatively short timescale of K2 observations for a majority of stars, and identify which stars are the host stars, which is particularly necessary in crowded fields. For candidate planets, we use the transit color (optical from K2 versus near-infrared from WIYN) to determine whether candidates have a planetary nature or are
instead false positives due to a stellar blend. This project ultimately contributes to a catalog of well-characterized K2 planets that could be prime targets for atmospheric characterization with the James Webb Space Telescope. I will also discuss prospects for extending this program to follow up planets and candidates discovered by the Transiting Exoplanet Survey Satellite.
Knicole Colón came to NASA Goddard Space Flight Center in early 2017 from NASA Ames Research Center in California, where she worked on the
Kepler and K2 missions. At Goddard, she is the Deputy Operations Project Scientist for the Hubble Space Telescope and the Deputy Director for the Transiting Exoplanet Survey Satellite (TESS) Science Support Center. Previously, she earned her Bachelor of Science in Physics from The College of New Jersey and her PhD in Astronomy from the University of Florida. She is an expert in the search for and characterization of transiting exoplanets using a variety of ground- and space-based telescopes. Plavchan
12/7/2018Friday3:00 PMEXPL L003
Matthew Rosenberger
Last week of semester
Patrick Vora
12/14/2018FridayFinal exams - No talk
12/21/2018FridayWinter break
12/28/2018FridayWinter break
1/4/2019FridayWinter break
1/11/2019FridayWinter break
1/18/2019FridayWinter break
1/25/2019Friday3:00 PM
Planetary 129
Igor MazinNRLConventional high(room)-temperature superconductivity: from A15 to MgB2 to H3S I will review the history of the half-century long quest for the room-temperature superconductivity, concentrating on the conventional electron-phonon mechanism. I will outline several stages, characterized by different paradigms, which can be tagged in a Potterian way thus:
(1) A-15 and the concept of an upper bound on Tc
(2) Lattice stability and the concept of a negative dielectric function
(3) MgB2 and the concept of doped covalent bonds
(4) H3S and the room temperature superconductivity (if the room is in Antarctica)
(5) Metallic hydrogen and LaH10 (regular-room temperature superconductivity)
First week of semester
Yuri Mishin
2/1/2019Friday3:00 PM
Planetary 129
Xu YiUVaPeter Plavchan
2/8/2019Friday3:00 PM
Planetary 129
Branislav NikolicU DelawareMultiscale quantum-classical micromagnetics: Fundamentals and applications in spintronicsThis talk introduces recently developed [1,2] multiscale and self-consistent computational tool which combines time-dependent nonequilibrium Green function (TD-NEGF) algorithms, scaling linearly in the number of time steps and describing quantum-mechanically conduction electrons in the presence of time-dependent fields of arbitrary strength or frequency, with classical description of the dynamics of local magnetic moments based on the Landau-Lifshitz-Gilbert (LLG) equation. Such TD-NEGF+LLG approach can be applied to a variety of problems where current-driven spin torque induces the dynamics of magnetic moments as the key resource for next generation spintronics. Previous approaches for describing such nonequilibrium many-body system have neglected noncommutativity of quantum Hamiltonian of conduction electrons at different times and, therefore, the impact of time-dependent magnetic moments on electrons which can lead to pumping of spin and charge currents that, in turn, can self-consistently affect the dynamics of magnetic moments themselves including introduction of non-Markovian damping and magnetic inertia terms into the LLG equation [2]. Thus, TD-NEGF+LLG can be viewed as “quantum-classical micromagnetics” which captures numerous effects missed by widely utilized purely classical micromagnetics. We use examples of current- or magnetic-field-driven motion of domain walls within magnetic nanowires (including their annihilation observed in very recent experiments [3]) to illustrate novel insights that can be extracted from TD-NEGF+LLG simulations. In particular, TD-NEGF+LLG as a nonperturbative (i.e., numerically exact) framework allows us to establish the limits of validity of simpler theories, such as the so-called spin-motive force theory [4] for pumped charge current by time-dependent noncollinear and noncollinear magnetic textures which turns out to be just the lowest order of the result predicted by TD-NEGF+LLG.


[1] M. Petrović, B. S. Popescu, U. Bajpai, P. Plecháč, and B. K. Nikolić, Phys. Rev. Applied 10, 054038 (2018).

[2] U. Bajpai and B. K. Nikolić,
[3] S. Woo, T. Delaney, and G. S. D. Beach, Nat. Phys. 13, 448 (2017).

[4] S. E. Barnes and S. Maekawa, Phys. Rev. Lett. 98, 246601 (2007).
Predrag Nikolic
3:00 PM
Lakshmi Narayanan Viswanathan
Erdal Yigit
2/22/2019Friday3:00 PM
Planetary 129
Joseph PoonUVaHoward Sheng
3/1/2019Friday3:00 PM
Planetary 129
JC Holbrook
U Western Cape
Astrophysics & Discrimination
Since 2009, I have been interviewing astrophysicists and people connected to astrophysics with a focus on collecting narratives about lived experiences. Focusing just on the experiences of Africans and African Americans, negative experiences are repeated in several institutions in different countries pointing to a larger culture of astrophysics that discriminates against this population. Highlighting these lived experiences benefits this population that without this information considered their experience to be unique, and allows for the astrophysics community to discuss actions to eliminate these negative behaviors.
Jarita Holbrook is an Associate Professor of Physics & Astronomy at the University of the Western Cape, South Africa. Her doctorate is in Astronomy & Astrophysics from the University of California, Santa Cruz. Her BS is in Physics from the California Institute of Technology and her MS is in Astronomy from San Diego State University. She was a NSF postdoctoral fellow at the Center for the Cultural Studies of Science, Technology, and Medicine, and at the Max Planck Institute for the History of Science. Holbrook works at the intersection of astronomy, indigenous knowledge, culture, and society in Africa including gender and diversity in STEM.
Jessica Rosenberg
3/15/2019FridaySpring break - no talk
3/22/2019Friday3:00 PM
Planetary 129
Gia-Wei ChernUVaErhai Zhao
3/29/2019Friday3:00 PM
Planetary 129
Jeff LynnNISTNirmal Ghimire
4/5/2019Friday3:00 PM
Planetary 129
Steph LaMassa
Space Telescope Science Institute
JWSTShobita Satyapal
4/19/2019Friday3:00 PM
Planetary 129
Karl Staplefeldt (tentative)
NASA JPLNASA Exoplanet Exploration ProgramEaster - Good FridayPeter Plavchan
4/26/2019Friday3:00 PM
Planetary 129
Doug MedlinSandia
Atomic structure of line-defects in crystals
Yuri Mishin
5/3/2019Friday3:00 PM
Planetary 129
Jillian Bellovary (tentative)
American Museum of Natural History / CUNY
Last week of semester
Shobita Satyapal
5/10/2019FridayFinal exams - No talk
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