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Events: Departmental Colloquia

  • A New Approach to the Cosmic Lithium Problem

    Guest: J. Christopher Howk, Department of Physics, University of Notre Dame
    Thursday, May 5, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    The predictions of light element abundances in standard Big Bang Nucleosynthesis agree very well with astrophysical probes of primordial material, with the exception of lithium. Most of the observational constraints we have on the primordial abundance and cosmic evolution of Li comes by way of the Li abundance in stellar atmospheres, which are four times lower than BBN predictions in the Planck era. A broad range of potential solutions to this "lithium problem" have been suggested, from stellar astrophysics solutions (depletion of the surface Li abundances in stars) to physics beyond the Standard Model (annihilating or decaying dark matter in the epoch of BBN). We have adopted a new approach to this problem, using observations of Li in interstellar gas of low-metallicity galaxies to probe the cosmic evolution of Li. I will summarize our results using this approach, including new estimates of the 7Li/6Li ratio that show no evidence for non-standard model approaches.

  • Unipolar quantum optolectronics

    Guest: Carlo Sirtori, Professor of Physics, Holder of the ENS-THALES Chair Ecole normale supérieure
    Thursday, April 14, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Highly sensitive and ultrafast optoelectronic systems operating in the atmospheric thermal window (8–14μm) are required for free-space communications, light detection and ranging (Lidar) and for high resolution spectroscopy and in observational astronomy.

    Unipolar quantum optoelectronics is a field of research of quantum physics that encompass an ensemble of semi-conductor devices aiming to develop enabling systems for applications in the atmospheric thermal window (8–14μm). The new generation of these devices is embedded into metamaterials for increasing their performances. They operate at room temperature with bandwidths of tens of GHz, ideal for free-space communications, light detection and ranging (Lidar), high resolution spectroscopy and observational astronomy.

  • Visualizing bending of RNA structures by a bound protein using Contrast Variation Small-Angle X-ray Scattering (CV-SAXS)

    Guest: Suzette A. Pabit, School of Applied and Engineering Physics, Cornell University, Ithaca, New York
    Thursday, March 31, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    RNA molecules play vital roles in cellular functions, not only as facilitators of the transcription of the genetic code carried by DNA but also as non-coding molecules that promote post-transcriptional regulation of gene expression, among other functions. It is necessary to develop biophysical methods that allow visualization of the structural dynamics of non-coding RNA molecules as part of large processing complexes involving protein partners. Here, we describe how to utilize small angle x-ray scattering (SAXS) with absolute calibration and contrast variation (CV-SAXS) to detect conformational changes of a primary microRNA upon binding with a part of the microprocessor complex. This method reports only on the RNA conformation within the complex and suggests that the protein bends the RNA. Supporting work using single molecule Fluorescence Energy Resonance Transfer (FRET) to study the conformation of RNA duplexes bound to the protein also shows bending. Together, these studies elucidate the role of the protein DGCR8 in interacting with RNA during the early stages of microRNA processing. This talk will also give a broad introduction into CV-SAXS and its different applications in the study of structural dynamics of protein-nucleic acid complexes.

  • A perspective on petahertz (nano)electronics

    Guest: Matthias Kling, Professor of Photon Science and (by courtesy) of Applied Physics Director of Science and R&D Division, LCLS, SLAC Stanford University
    Thursday, March 24, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Ultrafast electronic dynamics of solid-state materials, particularly under light excitation, are of great interests both fundamentally and practically due to the wide applications of optoelectronic devices, such as transistors, photovoltaics, or photodetectors. With the ability to control electronic dynamics in solids on attosecond time scales, the development of lightwave electronics holds promise for realizing ultrafast signal processing devices at frequencies up to the petahertz regime, surpassing current technology by orders of magnitude.

    The development of light-driven petahertz electronics is inherently connected to solid state nanophysics. First, the natural length scale of electron motion on the few attosecond time scale is on the order of one nanometer. Second, for the development of petahertz integrated circuits, the devices have to be both on nanometer length scales and be based on non-resistive processes, such as ballistic electron transport. Nanomaterials or nanostructured solids thus play a crucial role in the development of future on-chip petahertz electronics devices.

    The talk will give a broader introduction into this field and summarize a few recent results from our group. A perspective will be given on the next steps towards the implementation of petahertz signal processing.

  • Time-resolved optical microscopies of nanomaterials

    Guest: Prof. Dr. Achim Hartschuh, Department of Chemistry and Center for Nanoscience, LMU Munich, Munich, Germany
    Thursday, March 17, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Time-resolved optical spectroscopic techniques provide direct insight into the excited state dynamics and energies of materials. Implemented in a microscopy platform, these techniques can be utilized to establish spatio-temporal maps and to visualize photo-induced phenomena. In this talk, I will discuss three different implementations developed to address particular sample properties. First, we use time-resolved photoluminescence (PL) microscopy to visualize charge carrier diffusion in hybrid halide thin films. We implemented a scheme in which the detection point is scanned with respect to the excitation point. Pulsed laser excitation in combination with time-resolved detection then allows for the observation of charrier transport within the film. In the second example, we implemented both transient absorption and time-resolved PL microscopy to study the ultrafast dynamics of excitons in single semiconducting carbon nanotubes. This allowed us to formulate a unified model by combining unimolecular exciton decay and ultrafast exciton-exciton annihilation on a time-scale reaching down to 200 fs. In the third implementation, we explore the non-linear response of plasmonic nanoantennas using phase-shaping of broadband laser pulses. We find that the enhancement provided by resonant nanoantennas can be predicted from a complex frequency dependent field enhancement factor. We then demonstrate that the spectral phase associated with this factor can be utilized for phase-selective confocal imaging.

  • Planets, moons, and rings from the James Webb Space Telescope during Guaranteed Time Observations

    Guest: Dr. Naomi Rowe-Gurney, NASA GSFC [Howard]
    Thursday, March 3, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Now that Webb has launched, successfully deployed, and reached its destination at L2, we can outline the plans for the James Webb Space Telescope (JWST) guaranteed time observations (GTO). Heidi Hammel’s cycle 1 GTO programs focused on Solar System science include detailed time-resolved compositional mapping of the Martian surface, short- and medium-term climate monitoring of the giant planets, and exploratory spectroscopic studies of the full menagerie of asteroids and comets, from near-Earth objects to the Kuiper belt. These programs are designed to be benchmark observations for the JWST mission. In this talk I will introduce JWST, its capabilities for the Solar System and some of the science it will accomplish during its first year of operation. The focus will be on planetary systems with Ice Giant atmospheres as a detailed example. As a queer, Black woman I will also share some of the Equity, Diversity and Inclusion work I have done over my career.

  • Near-field microscopy for nanoscale materials characterization

    Guest: Dr. Joanna Atkin, Department of Chemistry University of North Carolina
    Thursday, February 17, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Semiconducting nanostructures have been proposed as material platforms for a wide variety of photonic, electronic, and photovoltaic elements. In order to realize these applications, careful design and characterization of electronic properties such as dopant concentration, activation, and distribution are needed. I will discuss the use of near-field optical microscopy as a non-destructive method for chemical, structural, and electronic imaging in nanomaterials, focusing on a specific application, the study of axially-doped silicon nanowires (SiNWs). We can detect local changes in the electrically-active doping concentration from the free-carrier absorption in both n- type and p-type doped SiNWs. The < 20 nm spatial resolution allows us to directly measure dopant transition abruptness and charge carrier properties in the vicinity of interfaces in single and multi-junction SiNWs, both in the infrared and the microwave spectral regimes. However, the tip is perturbative in terms of both the electromagnetic wave (frequency-resolved) and electrostatic (charge carrier redistribution) interactions, and this affects the measured results, an important consideration in nanostructured materials especially. Our results demonstrate the utility of near-field spectroscopy in probing local properties of nanomaterials, but emphasize the little-understood convolutional role of the tip in many forms of scanning probe microscopy.

  • Joint Theoretical and Experimental Efforts to Create New Atomic Data for Astrophysics

    Guest: Michael R Fogle Jr, PhD, Auburn University | Dept. of Physics
    Thursday, February 10, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Almost everything we know about the Universe has been discovered through the light that reaches us from the stars, galaxies, nebulae, and other astrophysical objects. The use of spectroscopy to analyze this light has yielded information about size, temperature, composition and dynamics of a wide array of astrophysical objects from comets to planetary nebulae. No field of science places higher demands on the quantity and accuracy of atomic data than astrophysics. This data is produced by Herculean efforts of both theorists and experimentalist, but both are often needed to overcome the shortcomings of the other. This talk will discuss the aspects of some new, collaborative, joint theoretical and experimental projects to produce new atomic data that is improved for analysis and modeling of data from the increasing observational capacity of our most advanced ground- and space-based telescopes.

  • Self-Organization of Lifelike Behaviors

    Guest: Dr. Jeremy England, Georgia Institute of Technology School of Physics
    Thursday, February 3, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

  • Effect of Processing on the Electrical Properties and Microstructure of Multiwalled Carbon Nanotube Films and Polymer Composites

    Guest: Prof. Rosario A. Gerhardt, School of Materials Science & Engineering Georgia Institute of Technology
    Thursday, January 20, 2022 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Multiwalled carbon nanotubes (MWNT) with dimensions of 8-15 nm and 0.5-2 μm long were used to make a series of CNT films via vacuum filtration, spin coating and inkjet printing. The method of deposition as well as the substrate used (filter paper of different sizes, photopaper, glass) were found to have an influence on the arrangement of the nanotubes and their resultant electrical response. Using a combination of impedance spectroscopy, equivalent circuit modeling, and microscopy techniques, it was possible to describe in detail how the electrical properties change as a function of how the MWNTs are distributed on the porous substrates by varying the number of deposited layers (1-20) as well as the dispersion concentration (0.1 to 5 mg/mL) using the vacuum filtration method. In the in-plane, four different electrical responses were observed and modeled: (1) a substrate dominated spectrum representing unconnected MWNTs, (2) one that included bundle and junction responses as well as some inductances representing sparsely distributed MWNT networks, (3) followed by a parallel RL circuit for partially connected MWNT networks and (4) finally a series RL circuit for fully connected MWNT networks. In the thru-plane, only two different electrical responses were observed and modeled. The results for the in- plane and thru-plane properties were used to generate percolation curves that show that electrical conductivity can change as much as 10 orders of magnitude for the same exact MWNTs. Similar experiments were also carried out for films that were spin coated and inkjet printed. The specific nanotube arrangements and electrical response depend on the number of layers deposited but in principle the trends observed are the same given that we used the SAME starting nanotube materials. Our results indicate that not only do the characteristics of the nanotubes themselves play a role but also the structure of the underlying substrate and the details of how the films are deposited.

  • Overview of Tritium Fuel Cycle Research at Savannah River National Laboratory

    Guest: Dr. George Larsen, Savannah River National Laboratory
    Thursday, November 18, 2021 3:55 pm - 4:55 pm
    Location: Physics Auditorium (202) and Zoom

    Recent developments, along with significant private investment, has spurred increased interest in fusion as a carbon-free source of energy. Hydrogen isotopes are the most promising fuel for a fusion reactor, but the development of the associated fuel cycle presents unique challenges and opportunities. Tritium, in particular, produces additional complexity due to its scarcity and radioactivity. In this talk, I provide an overview of recent and ongoing tritium fuel cycle research at SRNL in support of fusion energy. In addition, the recent management transition at SRNL includes strategic partnerships with regional universities in Georgia and South Carolina, including the University of Georgia. Therefore, I will also provide a brief overview of SRNL, opportunities for student internships, and highlight areas of potential collaboration.

    Zoom link: https://zoom.us/j/98442775233?pwd=WVc3aHl6UmxSUVlTZDNVdStDRWxzQT09

  • Scaling down the laws of thermodynamics

    Guest: Prof. C. Jarzynski, Department of Physics University of Maryland, College Park
    Thursday, November 4, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Thermodynamics provides a robust conceptual framework and set of laws that govern the exchange of energy and matter. Although these laws were originally articulated for macroscopic objects, nanoscale systems also exhibit “thermodynamic- like” behavior – for instance, biomolecular motors convert chemical fuel into mechanical work, and single molecules exhibit hysteresis when manipulated using optical tweezers. To what extent can the laws of thermodynamics be scaled down to apply to individual microscopic systems, and what new features emerge at the nanoscale? I will describe some of the challenges and recent progress – both theoretical and experimental – associated with addressing these questions. Along the way, my talk will touch on non-equilibrium fluctuations, “violations” of the second law, the thermodynamic arrow of time, nanoscale feedback control, strong system-environment coupling, and quantum thermodynamics.

  • Collapse of the Collapse: Physicists Return to Reality

    Guest: Prof. Murray Daw, Department of Physics and Astronomy Clemson University
    Thursday, October 28, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    We note the recent demise of the collapse hypothesis, that an integral part of quantum mechanics is the "collapse" of the state of a system when measured. Using recent work of Anthony Rizzi, we show that the Ensemble Interpretation provides a simple and natural resolution to the problem of measurement in quantum mechanics. Along the way, we give a fuller explanation of the Ensemble Interpretation (in spite of the familiar-sounding name, this is almost entirely unknown), measurement theory in general (correcting many common misconceptions), the resolution of the Schrodinger Cat experiment, Wigner’s Friend experiment, and the Extended Wigner’s Friend experiment. At the end we encourage discussion of how to deal with the pedagogical situation, given that almost all current QM textbooks are based on the (now defunct) collapse hypothesis.

  • Rydberg impurity in a quantum gas: a molecular Lego with quantum statistics

    Guest: Prof. H. R. Sadeghpour, Department of Astronomy Harvard University & Smithsonian Observatory
    Thursday, October 21, 2021 3:55 pm - 4:55 pm
    Location: Physics Auditorium (202) and Zoom

    Impurity physics is a celebrated sub-discipline in condensed matter. Spectroscopy of impurity dynamics in atomic quantum Bose and Fermi gases has invigorated this line of research. Here, I will describe steps we've taken toward understanding how Rydberg quenches in such gases probe quantum correlations and quantum statistics, how oligomeric Rydberg molecules evolve to many-body dressed states and how Fermi statistics inhibits chemical reactivity. I will also touch upon cases when impurity excitations are not isotropic with implications for engineering chiral interactions.

  • Breaking Speed and Resolution Limitations of AFM

    Guest: Prof. Simon Scheuring, Weill Cornell Medicine, Department of Anesthesiology, Department of Physiology and Biophysics, New York
    Thursday, October 14, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    High-speed atomic force microscopy (HS-AFM) is a powerful technique that provides dynamic movies of biomolecules at work. We successfully used HS-AFM to take movies – and determine dynamic parameters – of membrane trafficking systems such as ESCRT-III and clathrin, transporters and channels.

    To break current temporal limitations to characterize molecular dynamics using HS-AFM, we developed HS-AFM height spectroscopy (HS-AFM-HS), a technique whereby we oscillate the HS-AFM tip at a fixed position and detect the motions of the molecules under the tip. This gives sub- nanometer spatial resolution combined with microseconds temporal resolution of molecular fluctuations. HS-AFM-HS can be used in conjunction with HS-AFM imaging modes, thus giving access to a wide dynamic range.

    To break current resolution limitations, we developed Localization AFM (LAFM). By applying localization image reconstruction algorithms to peak positions in high-speed AFM and conventional AFM data, we increase the resolution beyond the limits set by the tip radius and reach quasi-atomic resolution on soft protein surfaces in native and dynamic conditions. The LAFM method allows the calculation of high-resolution maps from either images of many molecules or many images of a single molecule acquired over time, opening new avenues for single molecule structural analysis.

  • Chronicles of the Unknown: Webb Science across the Universe into the Solar System

    Guest: Dr. Stefanie N. Milam, NASA Goddard Space Flight Center, Astrochemistry Laboratory
    Thursday, September 30, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    In late 2021, scientific innovation will take the next step into space exploration and journey. The James Webb Space Telescope (sometimes called JWST or Webb), an orbiting infrared observatory that will complement and extend the discoveries of the Hubble Space Telescope but with longer wavelength coverage and greatly improved sensitivity, will launch into space on an Ariane 5 rocket from French Guiana. Webb will be the premier observatory of the next decade, serving thousands of astronomers and planetary scientist worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System. The discovery space for small bodies in the Solar System with Webb is unprecedented and will reveal new insights to planetesimal formation, composition, and distribution. Physically characterizing planetesimals throughout the solar system is often challenging via remote observations because of their combination of dark surfaces, extreme distances, and some diagnostic spectral features only distinguishable beyond Earth’s atmosphere. JWST's discoveries for these small bodies are expected to reveal the presence of previously unseen molecular gases and ices, constrain their physical state (e.g., crystalline phase and grain-size), and measure isotopic ratios of volatile elements (H, O, C, N). Additionally, imaging at longer wavelengths can be used to study temperature variations on larger bodies (Parker et al. 2016). In this talk, I will provide a status update of the Webb Telescope, briefly review the main science themes for JWST, and conclude with anticipated science from JWST's exploration of the objects in our Solar System, namely small bodies.

  • The Local Gaseous Universe

    Guest: Prof. Mary Putman, Department of Physics and Astronomy, Columbia University
    Thursday, September 23, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Most of the normal matter in the universe is in gaseous form. Stars form in galaxies when some of this gas settles into the central regions of their potential wells. In this talk, I will discuss the distribution of gas in and around galaxies in the nearby universe using ultraviolet absorption line and hydrogen emission line observations. The gas surrounding the Milky Way will be compared to other galaxies, and the gas within dwarf galaxies, the smallest galaxies in the universe, will be considered in the context of the gas surrounding larger galaxies. The flow of gaseous matter dictates the structure and fate of all types of galaxies.

  • Research Grant Proposals Reviewers Dynamics

    Guest: Prof. Burtrand I. Lee, PhD., FACerS, Office Research Grants, American Chemical Society
    Thursday, September 16, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    As we all are concerned with maintaining stable research support, we seek the best strategy for winning competitive research grants. It becomes imperative to better understand the evaluation process. While in the midst of the ever-increasing competition, the most popular evaluation means is peer review. Such basic questions as how the peer review is carried out, how the reviewers selected, what do the reviewers think when they encounter a research grant proposal to review, and what they like/don’t like are some of the questions that will be discussed. Other pertinent questions are expected to come from the listeners and participants.

  • Gas Flows in the Milky Way Halo

    Guest: Prof. Andrew Fox, Space Telescope Science Institute
    Thursday, September 2, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Gas flows play crucial roles in galaxy evolution. Inflowing gas provides fuel for star formation, whereas outflowing gas carries away the products of earlier generations of stars. In the Milky Way, we have a front-row seat for viewing the multi-phase gas flows that circulate material from the disk to the halo and back. I will focus on two prominent examples of gas flows: the Fermi Bubbles, a giant pair of outflowing lobes surrounding the Galactic Center, and the Magellanic Stream, an interwoven tail of filaments trailing the Magellanic Clouds as they orbit the Milky Way. I will discuss recent ultraviolet observations from the Hubble Space Telescope, which have provided new insight into the origin of the Fermi Bubbles and the Magellanic Stream. These observations have helped reveal the mechanisms by which the Milky Way exchanges mass with its surroundings and fuels its ongoing star formation.

    Related Papers:

    1) The Magellanic Stream: Circumnavigating the Galaxy
    https://ui.adsabs.harvard.edu/abs/2016ARA%26A..54..363D/abstract

    2) Probing the Fermi Bubbles in Ultraviolet Absorption: A Spectroscopic Signature of the Milky Way's Biconical Nuclear Outflow
    https://ui.adsabs.harvard.edu/abs/2015ApJ...799L...7F/abstract

  • Using information to learn what matters

    Guest: Prof. Ilya Nemenman, Department of Physics, Emory University
    Thursday, August 26, 2021 3:30 pm - 4:30 pm
    Location: Physics Auditorium (202) and Zoom

    Information theory was formulated by Shannon as the theory of communication. However, it has become a popular tool for answering a related — but a different — set of questions: namely, which features of a system are essential for its description, and therefore, must be preserved in a model describing it? These tools were developed largely in the context of biophysical systems, where traditional theoretical physics toolbox is insufficient to identify the relevant terms. I will walk us through a series of examples in different experimental biological and synthetic systems, where information theory has been used in this way.

  • Water on the Lunar Surface: To be or not to be?

    Guest: Dr. Thomas M. Orlando, Georgia Institute of Technology
    Thursday, April 15, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    The sources of molecular water in planetary systems is a subject of general interest in astrophysics and astrochemistry and its presence and persistence are critical for space missions involving long term human exploration. The Moon is the nearest exploration target and sources of water include primordial water, delivery via comets and meteorites, formation and release during small impact events, and solar wind interactions. An additional source term of molecular water is the thermally activated process known as recombinative desorption (RD) or associative desorption (AD) from lunar regolith grains. This involves hydroxyl (-OH) defects that were made by implantation of solar wind protons. Using several Apollo lunar samples, temperature program desorption (TPD) experiments conducted under ultra-high vacuum conditions yielded first order activation energies for desorption of chemisorbed molecular water and second order activation energies for the RD mediated formation and release of molecular water. Depending on the temperature excursions, RD can occur on a diurnal basis and is likely prevalent during impacts with meteorites and meteoroids. Once formed, the water can either desorb, or be transported on and within the regolith. Our combined experimental and modeling effort has successfully simulated recent observational data. Water formation via RD is likely general under astrophysical conditions that involve proton bombardment followed by thermal excursions ( > 400 K), and is critical to developing strategies for extraction of water for future, sustainable human space exploration missions.

  • Quantum Entanglement: Applications in Communication and Cryptography

    Guest: Dr. Mark M. Wilde, Louisiana State University
    Thursday, March 25, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Quantum entanglement is a key phenomenon that separates the classical and quantum theories of information. In this talk, I will begin by introducing the notion of entanglement and discuss its applications, beginning with fundamental protocols like teleportation, super-dense coding, and the CHSH game. Then I will progress to more sophisticated topics like various communication capacities of quantum channels and highlight the role of entanglement in each of them. Key references include https://arxiv.org/abs/1106.1445 and https://arxiv.org/abs/2011.04672.

    About the Speaker

    Mark M. Wilde is an Associate Professor in the Department of Physics and Astronomy and the Center for Computation and Technology at Louisiana State University. He is a recipient of the Career Development Award from the US National Science Foundation, co-recipient of the 2018 AHP-Birkhauser Prize from the journal Annales Henri Poincare, and Associate Editor for Quantum Information Theory at IEEE Transactions on Information Theory and New Journal of Physics. His current research interests are in quantum Shannon theory, quantum optical communication, quantum computational complexity theory, and quantum error correction.

  • Moire Superpotentials and Quantum Calligraphy of Single Photon Emitters in van der Waals Heterostructures

    Guest: Dr. Berend T. Jonker, Naval Research Laboratory, Washington DC
    Thursday, March 11, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Single photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantum-based technologies. A solid state host offers many advantages for realization of a functional system, but creation and placement of SPEs are difficult to control. We describe here a novel paradigm for encoding strain into 2D materials to create and deterministically place SPEs in arbitrary locations with nanometer-scale precision [1]. We demonstrate the direct writing of SPEs in 2D semiconductors based on a materials platform consisting of a WSe2 monolayer on a deformable substrate using an atomic force microscope nano-indentation process. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities and plasmonic structures.

    The weak interlayer bonding in van der Waals heterostructures (vdWh) enables one to rotate the layers at arbitrary azimuthal angles. For transition metal dichalcogenide vdWh, twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction has been observed to date, although reconstruction can be expected to have a significant impact on all measured properties, and its existence will fundamentally change our understanding of such systems. Here we demonstrate via conductive AFM and TEM that vdWh of MoSe2/WSe2 and MoS2/WS2 undergo significant atomic level reconstruction at twist angles ≤ 1° leading to discrete commensurate domains divided by narrow domain walls [2], rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior work. We show that this occurs because the energy gained from adopting low energy vertical stacking configurations is larger than the accompanying strain energy [3]. Such reconstruction impacts both the local conductivity and the optical properties.

    About the Speaker

    Berend T. Jonker is the Senior Scientist for Magnetoelectronic Materials in the Materials Science & Technology Division at the Naval Research Laboratory, Washington, DC.  His current research interests include spintronics, 2D materials, and topological materials.  Dr. Jonker is a Fellow of the American Physical Society (APS), the American Association for the Advancement of Science (AAAS), and of the American Vacuum Science Society (AVS).  He is the recipient of the Meritorious Presidential Rank Award, the NRL Hulburt Award, the Sigma Xi Award for Pure Science, several NRL Tech Transfer Awards, the Dolores M. Etter Navy Scientist Award, and others.  He has served as chair for the APS Topical Group on Magnetism, and for the AVS Magnetic Interfaces Division.  Dr. Jonker obtained his Ph.D. in solid state physics from the University of Maryland in 1983.  He has published over 270 articles in refereed journals, 5 book chapters, co-authored 17 patents, and presented over 140 invited lectures.

  • Controlling Correlations: Linear-, Nonlinear-, and Hydrodynamics in Quantum Materials

    Guest: Dr. Prineha Narang, Harvard University
    Thursday, March 4, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    The physics of quantum materials hosts spectacular excited-state and nonequilibrium effects, but many of these phenomena remain challenging to control and, consequently, technologically under-explored. My group’s research, therefore, focuses on how quantum systems behave, particularly away from equilibrium, and how we can harness these effects1. By creating predictive theoretical and computational approaches to study dynamics, decoherence and correlations in materials, our work could enable technologies that are inherently more powerful than their classical counterparts ranging from scalable quantum information processing and networks, to ultra-high efficiency optoelectronic and energy conversion systems. In this talk, I will present work from my research group on describing, from first principles, the microscopic dynamics, decoherence and optically-excited collective phenomena in quantum matter at finite temperature to quantitatively link predictions with 3D atomic-scale imaging, quantum spectroscopy, and macroscopic behavior. Capturing these dynamics poses unique theoretical and computational challenges. The simultaneous contribution of processes that occur on many time and length-scales have remained elusive for state-of- the-art calculations and model Hamiltonian approaches alike, necessitating the development of new methods in computational physics2–4. I will show selected examples of our approach in ab initio design of active defects in quantum materials5–7, and control of collective phenomena to link these active defects8–10. Building on this, in the second part of my seminar, I will show our predictions of linear and nonlinear dynamics and transport in Weyl semimetals11–14. I will discuss the anomalous landscape for electron hydrodynamics in systems beyond graphene, highlighting that previously- thought exotic fluid phenomena can exist in both two-dimensional and anisotropic three-dimensional materials15. Our work identifies phonon-mediated electron-electron interactions16–18 as critical in a microscopic understanding of hydrodynamics. Non-diffusive electron flow, and in particular electron hydrodynamics, has far-reaching implications in quantum materials science, as I will show in this talk. Finally, I will present an outlook on driving topological quantum materials far out-of-equilibrium to control the coupled degrees-of-freedom19,20.

    About the Speaker

    Prineha Narang is an Assistant Professor at the John A. Paulson School of Engineering and Applied Sciences at Harvard University. Prior to joining the faculty, Prineha came to Harvard as a Ziff Fellow and worked as a Research Scholar in Condensed Matter Theory at the MIT Department of Physics. She received an M.S. and Ph.D. in Applied Physics from the California Institute of Technology (Caltech). Prineha’s work has been recognized by many awards and special designations including a National Science Foundation CAREER Award in 2020, being named a Moore Inventor Fellow by the Gordon and Betty Moore Foundation, CIFAR Azrieli Global Scholar by the Canadian Institute for Advanced Research, a Top Innovator by MIT Tech Review (MIT TR35), and a Young Scientist by the World Economic Forum in 2018. In 2017, she was named by Forbes Magazine on their “30under30” list for her work in quantum science and engineering. Outside of science, she is an avid triathlete and runner. 

    Website: narang.seas.harvard.edu

  • Using machine learning to build robust interatomic potentials

    Guest: Dr. Kipton Barros, Los Alamos National Lab, Theoretical Division, Center for Nonlinear Studies
    Thursday, February 18, 2021 3:55 pm - 4:55 pm
    Location: Zoom Meeting

    Machine learning is emerging as a powerful tool for emulating electronic structure calculations. I will discuss recent work in building interatomic potentials relevant to chemistry, materials science, and biophysics applications. A key idea is active learning, in which the training data is iteratively collected to address weaknesses of the ML model. This approach can achieve a surprising level of transferability, as will be demonstrated with a case study for elemental aluminum.

    About the speaker:
    Kipton Barros has interests in computational physics, applied math, and computer science, and currently works in the Physics and Chemistry and Materials group at Los Alamos National Lab. A recent focus is applying machine learning to accelerate simulations and scientific discovery. Over the past few years, collaborations have spanned topics such as seismology, molecular dynamics simulation, fluid dynamics, and correlated electron physics.

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