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February 2018
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  1. CFNS Seminar

    4 pm, Small Seminar Room, Bldg. 510

    Hosted by: Andrey Tarasov and Maria Zurita

    We discuss how Electron Ion Collider measurements can be used to look for qualitative signals of the saturation phenomena. As an example, we study the scaling laws for exclusive vector meson production which is especially sensitive for the small-x gluons. We show that in the saturation framework we expect in general to see a significant change in the scaling exponents, given that we have large enough Q^2 lever arm that we can scan both the saturated and dilute regions. As a part of this study, the benefit of having also a large range of nuclear targets is emphasised. In addition to qualitative features, we discuss recent progress of developing the theory beyond leading order accuracy.

  2. CFN Colloquium

    4 pm, Bldg 735, CFN, Seminar Room, 2nd Floor

    Hosted by: Mircea Cotlet

    Electronic excitations are fundamental physical processes. Spectroscopic information, such as absorption and emission spectra, from electron or photon probes is crucial for materials characterization and interrogation, especially in the context of in situ studies of materials or processes under operando conditions. When experimental data are supplemented by first principles atomic modeling and state-of-the-art data analytics tools, a coherent physical picture emerges containing atomic level details of materials and insights derived from spectral signatures, which eventually allows us to establish the mechanistic understanding of the intriguing structure-property-function relationship. In this talk, the significance of the first principles modeling of electronic excitations is highlighted with three examples. In the first example, we investigated the oxygen 1s core-level binding energy shift of bilayer silica films on Ru(0001) in the X-ray photoelectron spectroscopy (XPS) measurement. Our study revealed that the binding energy shift is an electrostatic effect caused by the interplay of the surface and interface dipole moments. In the second example, we applied ab intio X-ray absorption near edge structure (XANES) modeling for spinel lithium titanate (Li4/3Ti5/3O4), an appealing lithium ion battery material. We identified key spectral features as fingerprints for quantitative assessment of the structural transformation during lithiation. In the third example, we demonstrated that how machine learning algorithms can be combined with XANES modeling to predict the 3D structures of metal nanoparticles on-the-fly.

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  1. MAR

    1

    Thursday

    CFN Colloquium

    "From band gaps to bound excitons: disentangling optical transitions and localized emitters in TMDCs at nanoscale dimensions"

    Presented by Jim Schuck, Associate Professor of Mechanical Engineering at Columbia University

    4 pm, CFN, Bldg 735, Seminar Room, 2nd Floor

    Thursday, March 1, 2018, 4:00 pm

    Hosted by: Matthew Sfeir

    The emergence of two-dimensional (2D) monolayer transition metal dichalcogenides (ML-TMDC) as direct bandgap semiconductors has rapidly accelerated the advancement of room temperature, 2D optoelectronic devices. Optical excitations on the TMDCs manifest from a hierarchy of electrically tunable, Coulombic free-carrier and excitonic many-body phenomena. Investigating the fundamental interactions underpinning these phenomena presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties. We show how optical detection of bound- and free-carrier photoexcitations is used to directly quantify carrier-induced changes of the quasiparticle band gap and exciton binding energies [1]. Pushing to the nanoscale, we demonstrate that a model hybrid architecture, a nano-optical antenna and a ML-WSe2 nanobubble, activates the optical activity of BX states at room temperature and under ambient conditions. These results show that engineered bound-exciton functionality as, in this case, localized nanoscale light sources, can be enabled by an architectural motif that combines localized strain and a nano-optical antenna, laying out a possible path for realizing room-temperature single-photon sources in high-quality 2D semiconductors.