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Welcome to Seminar in Physics (PHYS 703)
Welcome to Seminar in Physics (PHYS 703)
Fall 2020, George Mason University
[Aug 24, 2020 - Dec 16, 2020]
The course is offered online via Blackboard and Zoom.
The course is offered online via Blackboard and Zoom.
Where?
Where?
Online
When?
When?
Friday 15:30-16:45
Instructor
Instructor
Prof. Dr. Erdal Yiğit
Next Speaker & Abstract
Next Speaker & Abstract
Tentative Speaker Schedule
Tentative Speaker Schedule
Past Speakers & Abstracts
Past Speakers & Abstracts
Mark C. Hersam
Northwestern University
12/04/2020
Fundamentals and Applications of Mixed-dimensional Heterostructures
Layered two-dimensional (2D) materials interact primarily via van der Waals bonding, which has created new opportunities for heterostructures that are not constrained by epitaxial growth. However, it is important to acknowledge that van der Waals interactions are not limited to interplanar interactions in 2D materials. In principle, any passivated, dangling bond-free surface interacts with another via non-covalent forces. Consequently, layered 2D materials can be integrated with a diverse range of other materials, including those of different dimensionality, to form mixed-dimensional van der Waals heterostructures [1]. Furthermore, chemical functionalization provides additional opportunities for tailoring the properties of 2D materials [2] and the degree of coupling across heterointerfaces [3]. In order to efficiently explore the vast phase space for mixed-dimensional heterostructures, our laboratory employs solution-based additive assembly. In particular, constituent nanomaterials (e.g., carbon nanotubes, graphene, transition metal dichalcogenides, black phosphorus, boron nitride, and indium selenide) are isolated in solution, and then deposited into thin films with scalable additive manufacturing methods (e.g., inkjet, gravure, and screen printing) [4]. By achieving high levels of nanomaterial monodispersity and printing fidelity, a variety of electronic and energy applications can be enhanced including photodetectors, optical emitters, supercapacitors, and batteries [5-7]. Furthermore, by integrating multiple nanomaterials into heterostructures, unprecedented device function can be realized including anti-ambipolar transistors, gate-tunable Gaussian heterojunction transistors, and neuromorphic memtransistors [8-10]. In addition to technological implications for electronic and energy technologies, this talk will explore several fundamental issues including band alignment, doping, trap states, and charge/energy transfer across van der Waals heterointerfaces. [1] D. Jariwala, et al., Nature Materials, 16, 170 (2017).[2] S. Li., et al., ACS Nano, 14, 3509 (2020).[3] S. Padgaonkar, et al., Accounts of Chemical Research, 53, 763 (2020).[4] G. Hu, et al., Chemical Society Reviews, 47, 3265 (2018).[5] W. J. Hyun, et al., ACS Nano, 13, 9664 (2019).[6] W. J. Hyun, et al., Advanced Energy Materials, 10, 2002135 (2020).[7] K.-Y. Park, et al., Advanced Energy Materials, 10, 2001216 (2020).[8] M. E. Beck and M. C. Hersam, ACS Nano, 14, 6498 (2020).[9] M. E. Beck, et al., Nature Communications, 11, 1565 (2020).[10] V. K. Sangwan and M. C. Hersam, Nature Nanotechnology, 15, 517 (2020).Thomas Greathouse
Southwest Research Institute (SWRI), USA.
11/20/2020
A New View of Jupiter’s Aurora: The Juno UVS Perspective
Abstract: The ultraviolet spectrograph on board the Juno spacecraft (Juno UVS) is afforded a unique view of Jupiter’s polar auroras never before seen. Looking down from above each pole from altitudes ranging from ~0.05-7 RJ, UVS is able to see for the first time the behavior of the UV aurora at all local times. With 29 successful perijoves, or closest approaches, we have observed significant local time variability of all emissions from the main emission poleward. Additionally, we find the main emission in a variety of configurations from highly contracted intense emissions to expanded weak emissions. Surprising results include the apparent dependence of auroral polar emissions to magnetic local time on the night side as well as the high color ratio found in the previously defined swirl region, the inner portion of the northern auroral oval, compared to the low color ratio emissions found in the annulus between the main emission and the swirl region. In my talk I will describe the Juno UVS instrument, investigation, operational challenges and limitations, present highlights of data from the first 29 perijoves, and discuss the results thus far.
Figure 1) Jupiter's north pole aurora as seen from Juno UVS during the perijove 5 pass in March 2017. The brightness image on the left and color ratio image on the right are produced from 50 minutes of integrate scan data. The orange line coming close to the pole demarcates the terminator at the midpoint time of the observation (it is northern winter though Jupiter has only a 3 degree axial tilt). The red trace with times on the left plot (yellow on the right) shows the magnetic mapping of the Juno spacecraft position down to the surface of Jupiter. This view, similar to images from Hubble, shows the northern oval on the dayside. Attend the talk and see what the aurora looks like on the night side!
Heinz Krenn
Karl-Franzens-University of Graz, Institute of Physics, Austria
11/13/2020
Size does matter - from bulky to reduced dimensional magnets
Abstract: Nowadays the phrase „quantum materials” is well established in materials’ research. For ages, the origin of magnetism remained unexplainable, albeit magnetic materials were in practical use. It seems to be very remarkable that the intrinsic short-range quantum exchange length scales influence the bulk magnetic behavior, e.g. for creating soft or hard magnets. How the quantum length scales compete with microstructural length scales (grain sizes and boundaries) in the crossover from micro- to nano-crystallinity (0-, 1-, 2- dimensional to 3-dim bulky objects), is the main topic of this talk and treated in a tutorial-like manner. Magnetic blocking of superparamagnetic particles due to the crystal field anisotropy, exchange interaction and paramagnetic-relaxation are the ingredients of magnetic hysteresis loops, whereas quantum tunneling is decisive for molecular magnets. The phenomena are exclusively analyzed by SQUID-magnetometry. Bottom-up and top-down strategies are used for the fabrication of nanocrystalline materials, the focus is on the latter in applying severe plastic deformation (high pressure torsion). Not only the structural, but also the magnetic properties can be specifically tailored by this high-pressure method.
Figure 1
Libor Šmejkal
Johannes Gutenberg Universität, Germany & & Czech Academy of Sciences, Czech Republic
11/6/2020
Colossal spin splitting in a collinear antiferromagnet
Abstract: Energy bands of crystals encode quantum, topological and spintronics functionalities. Great interest was devoted to studies of time-reversal symmetry broken energy bands as they are candidates for low dissipation currents1. However, further progress is hindered by the lack of realistic systems with robust magnetic symmetry breaking. Here, the common collinear antiferromagnets were anticipated for many decades to be excluded from any macroscopic time-reversal symmetry breaking effects as they exhibit Kramers spin degenerate bands2. Our recent prediction of crystal time-reversal symmetry breaking by collinear anisotropic antiferromagnetism3 opens previously overlooked avenues.Figure 1 Colossal spin splitting in Ruthenium dioxide calculated from first principles.In the first part of the talk, we will discuss the basic properties of this new type of antiferromagnetic spin splitting, its local magnetic symmetry origin and symmetry criteria for its emergence. The anisotropic magnetisation densities result from a combination of collinear antiferromagnetism and nonmagnetic atoms. Our antiferromagnetic spin splitting properties starkly contrast those of conventional spin-orbit interaction spin splitting. It can preserve spin quantum number3, is of exchange origin reaching colossal eV values34 unparalleled in systems without magnetic dipole and can materialize in centrosymmetric potentials356.In the second part of our talk, we will discuss the experimental implications of our antiferromagnetic splitting. We will show that this antiferromagnetic spin splitting generates a crystal Hall effect which was recently observed in RuO237. We will show that the antiferromagnetic spin splitting effects are surprisingly strong. The spin spitting of more than 1eV predicted in RuO2 is larger than the record values reported in Rashba systems. Furthermore, we predict spin current angle in RuO2 to reach 34 degrees, a value three times larger than the largest spin current angles found in a high-throughput scan of 20 000 materials8. 1. Šmejkal, L., Mokrousov, Y., Yan, B. & MacDonald, A. H. Topological antiferromagnetic spintronics. Nat. Phys. 14, 242 (2018).2. Šmejkal, L., Železný, J., Sinova, J. & Jungwirth, T. Electric Control of Dirac Quasiparticles by Spin-Orbit Torque in an Antiferromagnet. Phys. Rev. Lett. 118, 106402 (2017).3. Šmejkal, L., González-Hernández, R., Jungwirth, T. & Sinova, J. Crystal time-reversal symmetry breaking and spontaneous Hall effect in collinear antiferromagnets. Sci. Adv. 6, eaaz8809 (2020).4. Ahn, K.-H., Hariki, A., Lee, K.-W. & Kuneš, J. Antiferromagnetism in RuO2 as d-wave Pomeranchuk instability. Phys. Rev. B 99, 184432 (2019).5. Hayami, S., Yanagi, Y. & Kusunose, H. Momentum-Dependent Spin Splitting by Collinear Antiferromagnetic Ordering. J. Phys. Soc. Japan 88, 123702 (2019).6. Yuan, L.-D., Wang, Z., Luo, J.-W., Rashba, E. I. & Zunger, A. Giant momentum-dependent spin splitting in centrosymmetric low-Z antiferromagnets. Phys. Rev. B 102, 014422 (2020).7. Feng, Z. et al. Observation of the Crystal Hall Effect in a Collinear Antiferromagnet. arxiv (2020).8. González-Hernández, R. et al. Efficient Electrical Spin-Splitter Based on Non-Relativistic Collinear Antiferromagnetism arxiv (2020).
Madeleine Phillips
NRL, USA
10/30/2020
Auger-assisted hot hole photocurrents in transition metal dichalcogenide/hexagonal boron nitride heterostructures
Abstract: Transition metal dichalcogenide (TMD) monolayers are two-dimensional semiconductors with an optical band gap that makes them attractive candidate components of quantum optoelectronic systems. Devices utilizing TMD monolayers frequently employ hexagonal boron nitride (hBN), a wide bandgap insulator, as both a substrate and a capping material to isolate the TMD layers both physically and electronically from other parts of the device. However, recent experiments show that under laser excitation, hBN-encapsulated MoSe2 and WSe2 exhibit hole currents that flow from the TMD through the hBN. In this talk, I will argue that these photocurrents originate from Auger recombination in the TMD monolayers, and I will discuss the potential for these photocurrents to be used as a probe of TMD monolayer properties.David Stevenson
Caltech, USA
10/23/2020
Juno at Jupiter
Abstract: Jupiter is in the class of planets known as gas giants— those planets primarily made from hydrogen and helium gas, which, under gravitational compression, become metallic fluids. It’s the most massive planet in our solar system— so massive that it influences everything else, including the delivery of material to Earth. Juno entered Jupiter orbit in 2016 and we are still collecting data. I'll tell you mostly about the three key measurements: the gravity field, the magnetic field and the passive microwave emission. Gravity has told us that there may be a diluted core , that is, the rock and ice component (only of order 5-10% of the total) is distributed outward rather than very centrally located. Gravity also tells us that the winds die off at about 3000km depth, deeper than purely meteorological but shallower that old ideas based on the size of the metallic region.The magnetic field is unlike Earth or what we thought before the mission, with a striking "Great Blue Spot" near the equator and more structure in the North than in the South.The microwave emission tells us about the atmosphere, but down to pressures of order a thousand times Earth's atmosphere. These data show a major surprise: the ammonia (and possibly the water) are not uniformly distributed at depth. Like any outstanding mission, Juno has surprised us and confounded our understanding of Jupiter.Noam Bernstein
U. S. Naval Research Laboratory
10/16/2020
Gaussian process regression for machine learning interatomic potentials with and without much human effort
Abstract: Interatomic potentials describe the interactions of atoms in a material as a function of the neighboring atoms, making it possible to simulate the material by following the atomic trajectories. While traditional potentials use functional forms that capture specific bonding physics, machine-learning methods for regression have made it possible to use nonparametric functional forms and consider the potential development as a more generic problem of fitting a smooth scalar energy function in a high dimensional space. However, because of their variational freedom, such potentials require large fitting datasets, which are typically developed using large amounts of manual selection and tuning of configurations by the researcher. After describing the general problem and how it relates to other applications of machine-learning in computational materials science, I will show a successful example of a Gaussian Approximation Potential developed for silicon using this type of a hand-tuned fitting database. Then I will describe an automated iterative procedure we have developed, based on random-structure search, to simultaneously explore configuration space and fit the potential. The method has been applied to several elemental materials with different bonding types, from insulators to metals, including boron with its complex icosahedral α-B12 structure. I will show how the process converges in a few iterations, and how the resulting potentials reproduce the reference DFT values on a number of bulk and defect properties. Finally, I'll discuss the possibility of developing universal heuristics for the descriptors we use, and how they will apply to potentials for systems with more than one chemical element.Dave Fritts
Global Atmospheric Technologies and Science (GATS), Boulder, CO
10/2/2020
New Insights into Turbulence Sources and Dynamics Enabled by High-Resolution Imaging and profiling of the Atmosphere at High Altitudes
Abstract: Atmospheric structure and variability from local to global scales depend in fundamental ways on turbulence occurring at the smallest scales of motion. Weather and climate prediction rely on suitable descriptions of turbulence sources and influences, but the lack of a quantitative understanding of these dynamics at present limits forecast accuracy and often imposes large personal and societal costs. Thus, an improved understanding of these dynamics is needed, and thin layers at very high altitudes that reveal fluid motions comprise perhaps the best natural laboratory. Recent advances in imaging resolution, and the occurrence of, and sensitivity to, very thin layers now allow tracking of structures and evolutions from source scales spanning 100 km or more and extending to the intermediate and smallest scales within the turbulence inertial range. High-performance computing is also able to address the relevant dynamics spanning more than 3 decades of scales, enabling comparisons with high-resolution observations and quantification of their influences. This presentation will employ high-resolution imaging of Polar Mesosphere Clouds (PMCs) viewed from the ground and by high-resolution imaging and lidar profiling aboard the NASA PMC Turbo long-duration balloon experiment that floated at 37 km from N. Sweden to NE Canada in July 2018. Related ground-based observations of similar dynamics from the Andes Lidar Observatory in Chile also aid interpretation of events that have been seen in the laboratory for ~40 years, but not explained to date, which can finally be diagnosed and quantified by remote atmospheric observations at very high altitudes and related modeling.Alexander S. Medvedev
Max Planck Institute for Solar System Research, Göttingen, Germany
9/25/2020
Mars studies with a general circulation model
Abstract: Mars is the second most studied planet in the Solar System. While the number of observations continuously increase, our understanding of the details of the Martian atmospheric processes depends heavily on modeling. The goal of the presentation is to familiarize you with studies of the Martian atmosphere performed with the Max Planck Institute Martian general circulation model. An outline of the existing models and how they can be applied to different atmospheric layers will be given. Specifics of processes at these levels and unresolved scientific questions will be discussed. The emphasis will be on how ongoing and future satellite measurements can help to resolve them in conjunction with modeling.Peter Plavchan & Faith M. Gaile
George Mason University, Fairfax , USA.
9/18/2020
Fostering an Inclusive Environment in our Department: Bias and Inclusion Training
Abstract: What does it mean, and what does it take, to foster an inclusive environment? We will cover definitions commonly used in bias and inclusion training. We will discuss real case studies that have directly contributed to the marginalization of underrepresented groups in our specific department. We will conclude with defining and discussing allyship, and present concrete actions to improve our department inclusiveness. Why should you care to attend? In one word, retention. Studies have shown that successful methods for inclusive interventions in STEM at the undergraduate academic career stage center around the concept of identity building. In other words, the more a student, staff or faculty member feels like they belong in the department - e.g. that their department or major is part of their identity - then the more likely it is that they will succeed. In the case of undergraduate students, identity association with a department/major is a predictor of retention and graduation rates.Chinmaya Nayak
Swedish Institute of Space Physics, Upsala, Sweden.
9/11/2020
Can a geomagnetic storm affect internal gravity waves in the middle atmosphere? A case study during August 25-26, 2018 geomagnetic storm
Abstract: The atmosphere-thermosphere-ionosphere system is a highly complicated system where the different regions are coupled through various forcings, both from below and above. The internal gravity waves play an important role in the coupling between the atmosphere-ionosphere system. The GWs which originate lower in the atmosphere propagate upwards carrying and redistributing energy and momentum along with them. On the other hand, geomagnetic storms are the perfect examples of forcing from above. Although the effects of geomagnetic storms in the ionosphere have been well documented, their effects on the lower atmosphere are scarcely studied. Geomagnetic storms are known to drive large scale waves in the ionosphere known as travelling ionospheric disturbances (TIDs). However, their effects on the GW propagation in the middle or lower atmosphere is completely unknown. In this talk we will consider the geomagnetic storm of August 25-26, 2018. The reason for choosing this particular storm is because it was the last major storm of solar cycle 24 and almost near solar minimum conditions when the atmosphere ionosphere coupling is expected to be more prominent. The GWs in the study are derived from the vertical temperature profiles obtained by the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on board NASA's TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite.Scott England
Virginia Tech, USA.
9/4/2020
Impacts of atmospheric tides on the composition of the upper atmosphere and density of the ionosphere
Abstract: Atmospheric waves are evident throughout Earth’s upper atmosphere. A particularly notable type of these waves are atmospheric tides, which can create global-scale modulations of the upper atmosphere. Features corresponding to these atmospheric tides are also found in observations of the ionosphere – the charged particle counterpart to our upper atmosphere. It is widely thought that the primary mechanism responsible for imprinting this signature of the atmospheric waves on the ionosphere involves the wind-driven dynamo, near the base of the upper atmosphere. However, both theory and modeling suggest that other mechanisms such as changes in the neutral atmosphere’s density and composition should also play a part. Providing observational confirmation of this has proved difficult, owing to the observations of the composition being influenced by changes in the ionosphere, that may be misinterpreted. Analogous observations at Mars have also revealed similar atmospheric waves, and corresponding ionospheric features. However, at Mars it is believed that modification of the atmospheric density and composition is the primary cause of the ionospheric signatures, and electrodynamics plays no role. The new ICON spacecraft, with coordinated observations of Earth’s upper atmosphere and ionosphere offers an opportunity to see if the same mechanism that is so dominant at Mars can also play a role at Earth.Short bio: Dr England received his PhD in 2005 at the University of Leicester, UK, with the title: “The role of gravity waves in coupled middle-upper atmosphere dynamics”. He worked as a Postdoc and researcher at UC Berkeley’s Space Sciences Lab from 2005 to 2016. England's research focuses on the interactions between planetary atmospheres and the space environment. He is the Project Scientist for NASA’s Ionospheric Connection Explorer (ICON), a co-Investigator on NASA’s Global-scale Observations of the Limb and Disk (GOLD) mission of opportunity and a Participating Scientist on NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission.
Announcement & Information
Announcement & Information
Summer 2020 calendar: https://registrar.gmu.edu/calendars/summer-2020/
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