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AUG
22
Thursday
Condensed-Matter Physics & Materials Science Seminar
"The 2-spinon contribution to the longitudinal structure factor in the XXZ model"
Presented by Isaac Perez Castillo, Institute of Physics, UNAM and London Mathematical Laboratory
1:30 pm, ISB Bldg. 734 Conf. Rm. 201 (upstairs)
Thursday, August 22, 2019, 1:30 pm
Hosted by: Alexei Tsvelik
In this work we derive exactly the two-spinon contribution to the longitudinal dynamical structure factor of the anisotropic Heisenberg spin-1/2 chain in the gapped regime by using quantum group approach. We will briefly discuss some of the mathematical difficulties when confronting form factor formulas given by quantum group approach and how to overcome these obstacles. We end up by contrasting our results with those coming from perturbation theory, while comparison to DMRG and experiments are currently underway.
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AUG
23
Friday
NSLS-II Friday Lunchtime Seminar
"Coherent X-ray Scattering Studies of Surface Processes: Self-Organized Ion Beam Nanopatterning"
Presented by Karl Ludwig, Boston University, MA
12 pm, NSLS-II Bldg. 743 Room 156
Friday, August 23, 2019, 12:00 pm
Hosted by: Ignace Jarrige
Karl Ludwig*, Mahsa Mokhtarzadeh*, Jeffrey Ulbrandt#, Peco Myint*, Suresh Narayanan+, and Randall Headrick# *Boston University / #University of Vermont +Advanced Photon Source, Argonne National Laboratory Enabled by the continued increases in brilliance available from synchrotron x-ray sources, our goal is to develop coherent x-ray scattering as a powerful new tool for the investigation of surface dynamics during growth and patterning processes. In particular, our research has been developed new approaches in X-ray Photon Correlation Spectroscopy (XPCS), using it to examine the dynamics of kinetic roughening in amorphous thin film growth, the step structure evolution in polycrystalline epitaxial growth and self-organized ion beam nanopatterning. Broad-beam low-energy ion bombardment can lead to the spontaneous formation of nanoscale surface structures, but the dominant mechanisms driving evolution remain controversial. Here I will describe our studies of the classic case of ion-beam rippling of SiO2 surfaces, in which we examine the relationship between the average kinetics of ripple formation and the underlying ?uctuation dynamics. In the early stage growth of ?uctuations we find a novel behavior with memory stretching back to the beginning of the bombardment. For a given length scale, correlation times do not grow signi?cantly beyond the characteristic time associated with the early-stage ripple growth. In the late stages of patterning, when the average surface structure on a given length scale is no longer evolving, dynamical processes continue on the surface. Nonlinear processes dominate at long length scales, leading to compressed exponential decay of the speckle correlation functions, while at short length scales the dynamics appears to approach a linear behavior consistent with viscous ?ow relaxation. This behavior is found to be consistent with simulations of a recent nonlinear growth model. In addition
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AUG
23
Friday
NT/RIKEN Seminar
"Symmetries in quantum field theory and quantum gravity"
Presented by Daniel Harlow, MIT
1:15 pm, Building 510, CFNS Seminar room 2-38
Friday, August 23, 2019, 1:15 pm
Hosted by: Niklas Mueller
It has long been suspected that symmetries in quantum gravity are highly constrained. In this talk I will describe joint work with Hirosi Ooguri, where we use the power of the AdS/CFT correspondence to prove three conjectures of this type: that there are no global symmetries, that there must be objects transforming in all representations of any gauge symmetry, and that any gauge group must be compact. Real world implications include the existence of magnetic monopoles and neutrinoless double beta decay, although we so far are unable to give estimates for when these should be seen. An important point, which we dwell on at length, is the proper definition of gauge and global symmetries in quantum field theory.
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AUG
26
Monday
Condensed-Matter Physics & Materials Science Seminar
"Tailoring the twinning of DyBa2Cu3O7-x thin films with atomic-layer-by-layer molecular beam epitaxy"
Presented by Daniel Putzky, Max Planck Institute for Solid State Research, Germany
1:30 pm, ISB Bldg. 734, Conf. Rm. 201 (upstairs)
Monday, August 26, 2019, 1:30 pm
Hosted by: Tony Valla/Ilya Drozdov
In this talk I will present the work on high-quality, epitaxial DyBa2Cu3O7-x (DBCO) thin films grown by molecular beam epitaxy (MBE). In contrast to the previous DBCO growth by MBE using co-deposition technique, we have employed an atomic-layer-by-layer shuttering sequence with in-situ RHEED feedback. Films grown on LSAT (100), NGO (110) and STO (100) have a sharp superconducting transition above 80 K. Scanning-transition electron microscopy (STEM) shows atomically sharp substrate-film interface and the absence of stacking faults, unlike films previously grown by PLD. In the second part of the talk I will focus on the structural investigation using x-ray diffraction (XRD). In-plane scans at the KARA synchrotron confirm the epitaxial relationship to the substrate. In addition the formation of twin domains with the bulk-like orthorhombic crystal structure were observed. By reducing the film thickness the tetragonal to orthorhombic phase transition can be suppressed while the films still remain superconducting.
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SEP
5
Thursday
Environmental & Climate Sciences Department Seminar
"Broadband Radiometry during EarthCARE: An Improved Solar Radiance-to-Flux Conversion"
Presented by Florian Tornow, Freie Universitat Berlin
11 am, Large Conference Room, Bldg. 490
Thursday, September 5, 2019, 11:00 am
Hosted by: Mike Jensen
Outgoing radiative fluxes at top-of-atmosphere (TOA) are driven by the interaction of clouds, aerosols, and radiation. The upcoming EarthCARE mission aims to improve our understanding of this interaction and compares shortwave (SW) and longwave TOA fluxes from (1) radiative transfer simulations – acting on cloud and aerosol properties retrieved from onboard active and passive instruments – with (2) flux estimates based on broadband radiometer (BBR) measured radiances over horizontal domains of 10x10 km. To improve BBR-based TOA SW flux estimates, this talk explores a new approach that incorporates additional parameters (cloud-top effective radius and cloud-topped water vapor) and produces significantly different flux estimates when compared against state-of-the-art methodology.
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SEP
6
Friday
CFN Colloquium
"Controlling Light Matter Interactions in Layered Materials with Conventional and Topological Band Structures"
Presented by Prof. Ritesh Agarwal, University of Pennsylvania
4 pm, CFN, Bldg 735, Seminar Room, 2nd Floor
Friday, September 6, 2019, 4:00 pm
Hosted by: Chang-Yong Nam
Strongly confined electrical, optical and thermal excitations drastically modify material's properties and break local symmetries that can enable precisely tunable novel responses and new functionalities. We will discuss the effect of engineered plasmonic lattice on light matter interactions in 2D excitonic crystals to produce novel responses such as enhanced and tunable emission, Fano resonances and strong exciton-plasmon polaritons, which can be precisely controlled by geometry and applied fields to produce new device concepts. Our recent work on collective polaritonic modes and the formation of a complete polaritonic bandgap in few-layered excitonic semiconductors coupled to plasmons will also be presented along with our ability to control them via externally applied electric fields. We will also discuss our efforts to explore the optoelectronic properties of MoxW1-xTe2, which are type-II Weyl semimetals, i.e., gapless topological states of matter with broken inversion and/or time reversal symmetry, which exhibit unconventional responses to externally applied fields. We have observed spatially dispersive circular photogalvanic effect (s-CPGE) over a wide spectral region (0.2 - 2.0 eV range) in these materials. This effect shows exclusively in the Weyl phase and vanishes upon temperature induced topological phase change. Since the photon energy leads to interband transitions between different electronic bands, we use the density matrix formalism to describe the photocurrent response under chiral optical excitation with a spatially inhomogeneous beam. We will discuss how spatially inhomogeneous optical excitation and unique symmetry and band structure of Weyl semimetals produces CPGE in these systems. The effect of band inversion, Berry curvature and asymmetric carrier relaxation in this material system on the s-CPGE signal will also be discussed along with the implications for designing new optoelectronic devices. Short B
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SEP
12
Thursday
Chemistry Department Colloquium
"Oxygen Catalysis for Large Scale Solar Energy Harvesting and Storage"
Presented by Dunwei Wang, Boston College
11 am, Hamilton Seminar Room, Bldg. 555
Thursday, September 12, 2019, 11:00 am
Hosted by: Anatoly Frenkel
: As we enter Anthropocene, it has become clearer than ever that a sustainable future will be one built on renewable energy resources. A critical challenge in realizing such a goal is to harvest and store renewable energy efficiently and inexpensively on a terawatt scale. Of the options that have been examined, using the energy to directly synthesize fuels stands out. When the renewable energy source is solar, the process is often referred to as artificial photosynthesis, highlighting the similarities with natural photosynthesis. Within this context, we have focused on understanding the detailed processes that are important to artificial photosynthesis. More specifically, a main thrust of our research has been water oxidation by photochemical reactions on the surface of inorganic materials. We strived to understand the detailed physical and chemical processes at the solid/liquid interface, with the goal of enabling facile electron extraction from water for the eventual proton reduction for hydrogen generation or the carbon dioxide reduction for the production of complex organic compounds. It was discovered that the light harvesting and catalytic components in an integrated system exerts profound influences on each other in a complex fashion. Detailed studies generated new insights into the water oxidation reactions at the molecular level, some of which was readily transferred to other reactions such as methane transformation. These efforts also inspired us to study oxygen catalysis in aprotic systems for applications with more immediate implications, such as metal air batteries.
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SEP
12
Thursday
Particle Physics Seminar
"Radar detection of neutrino-induced cascades in ice: experimental evidence and future prospects"
Presented by Steven Prohira, The Ohio State University
3 pm, Small Seminar Room, Bldg. 510
Thursday, September 12, 2019, 3:00 pm
Hosted by: Alessandro Tricoli
In order to detect ultra-high energy (UHE) neutrinos (? few PeV), tens to hundreds of cubic kilometers of material must be instrumented, owing to the exceedingly low flux. Radio methods have been suggested as the clear way forward in the UHE regime, owing to very long path lengths for radio waves in ice, meaning that a massive volume can be sparsely instrumented. Among radio techniques, the most recent—and most promising—is the radar detection method. Here, radio waves illuminate a volume, and if an UHE neutrino-induced cascade occurs within the volume, these waves are reflected to a distant receiver. In this seminar, we present the first evidence of detection of such a radar reflection, captured at SLAC in experiment T-576. We then present the science case for radar, and show that it has the best discovery potential for a detector technology in the UHE range.
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SEP
27
Friday
NSLS-II Friday Lunchtime Seminar
"Quantum Computing on crystalline beams of ions: the concept and proof-of-principle experiments"
Presented by Timur Shaftan, NSLS-II
12 pm, NSLS-II Bldg. 743 Room 156
Friday, September 27, 2019, 12:00 pm
Hosted by: Ignace Jarrige
One of the promising directions in Quantum Computers (QC) is based on using ion traps. In a modern QC, several tens of ions are collected in an electromagnetic trap of a cm in size, with their motion cooled down to micro K temperature level, leading to entanglement of their quantum states, controlled by laser and RF fields. These ions = qubits then used to run quantum computations at unprecedent rate using specialized codes (check, for example, QuTip, Quantum Toolbox in Python). I will discuss a concept of a QC, which holds a promise to support 105 of qubits in contrast to the state-of-the-art devices. The idea is to use crystalline beams of ions in an accelerator as the medium for qubits. The crystalline beams were demonstrated in storage rings in 1980s when many protons, being cooled with electron beam formed a revolving ring with crystalline-like structure inside. Marrying this concept with that of the QC on a conventional ion trap, one might consider expansion of the QC to a large particle accelerator with high qubit capacity. The latter is important for expansion of QC capabilities, including the processing power and robustness against errors due to decoherence. In this presentation I will go over the concept and my analysis of a few challenges that require proof-of-principal experiments so that the some basic aspects of this interesting concept are validated.
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OCT
10
Thursday
Environmental & Climate Sciences Department Seminar
"Progress in quantifying the global effective radiative forcing due to aerosol-cloud interactions"
Presented by Johannes Quaas, University of Leipzig
11 am, Large Conference Room, Bldg. 490
Thursday, October 10, 2019, 11:00 am
Hosted by: Steve Schwartz
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OCT
15
Tuesday
Chemistry Department Colloquium
"DESIGN OF METAL/METAL-OXIDE HETEROGENEOUS CATALYSTS FOR ENERGY AND CHEMICAL CONVERSION"
Presented by Eranda Nikolla, Wayne State University
11 am, Hamilton Seminar Room, Bldg. 555
Tuesday, October 15, 2019, 11:00 am
Hosted by: Sanjaya Senanayake
Dwindling fossil fuel resources and high levels of CO2 emissions have increased the need for renewable energy and efficient energy conversion and storage systems. In this talk, some of our recent work on designing efficient (active, selective and stable) catalytic systems for energy and chemical conversions will be discussed. First, I will highlight our work on designing layered nickelate oxide electrocatalysts for electrochemical oxygen reduction and evolution reactions. These processes play an important role in fuel cells, electrolyzers and Li-air batteries. We have utilized density functional theory (DFT) calculations to identify the factors that govern the activity of nickelate oxides toward these processes. Using a reverse microemulsion approach, we demonstrate an approach for synthesizing nanostructured nickelate oxide electrocatalysts with controlled surface structure. These nanostructures are thoroughly characterized using atomic-resolution high angle annular dark field (HAADF) imaging along with electron energy-loss spectroscopy (EELS) performed using an aberration corrected scanning transmission electron microscope (STEM). Controlled kinetic isotopic and electrochemical studies are used to develop structure/performance relationships to identify nickelate oxides with optimal electrocatalytic activity. Secondly, I will discuss our efforts on designing selective catalysts for biomass conversion processes. Development of active and selective catalysts for biomass conversion is critical in realizing a renewable platform for fuels and chemicals. I will highlight some of our recent work on utilizing reducible metal oxide encapsulated noble metal catalytic materials to promote hydrodeoxygenation (HDO) of biomass-derived compounds. We show enhancement in HDO activity and selectivity due to the encapsulation of the metal nanoparticles by an oxide film providing high interfacial contact between the metal and metal oxide sites, and restrictive acce
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OCT
28
Monday
Chemistry Department Colloquium
"Electronic Cooperativity in Supported Single and Multinuclear-Sites for Catalytic C-C and C-H Bond Functionalization"
Presented by Dr. Massimiliano Delferro, Argonne National Laboratory
11 am, Hamilton Seminar Room, Bldg. 555
Monday, October 28, 2019, 11:00 am
Hosted by: Sanjaya Senanayake
Systematic study of the interactions between organometallic catalysts and metal oxide support materials is essential for the realization of rational design in heterogeneous catalysis. In this talk, I will describe the stoichiometric and catalytic chemistry of a series of organometallic complex chemisorbed on a variety of metal oxides as a multifaceted probe for stereoelectronic communication between the support and organometallic center. Electrophilic bond activation was explored in the context of stoichiometric hydrogenolysis as well as catalytic hydrogenation, dehydrogenation, and H/D exchange. Strongly acidic modified metal oxides such as sulfated zirconia engender high levels of activity toward electrophilic bond activation of both sp2 and sp3 C–H bonds, including the rapid activation of methane at room temperature; however, the global trend for the supports studied here does not suggest a direct correlation between activity and surface Brønsted acidity, and more complex metal surface interactions are at play.
