2017-2018 Colloquia Abstracts

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Thursday, Sept. 7, 2017

Mapping Atomic Motions with Ultrabright Electrons: Realization of the Chemists’ Gedanken Experiment

R. J. Dwayne Miller, The Max Planck Institute for the Structure and Dynamics of Matter, the Hamburg Centre for Ultrafast Imaging, and Departments of Chemistry and Physics, University of Toronto
Mother Nature and the Big Bang of Chemistry

One of the dream experiments in chemistry has been to watch atomic motions on the primary timescales of chemistry. This prospect would provide a direct observation of the reaction forces, the very essence of chemistry, and the central unifying concept of transition states that links chemistry to biology. This experiment has been referred to as "making the molecular movie". Due to the extraordinary requirements for simultaneous spatial and temporal resolution, it was thought to be an impossible quest and has been previously discussed in the context of the purest form of a Gedanken experiment. With the recent development of ultrabright electron sources capable of literally lighting up atomic motions, this experiment has been realized (Siwick et al. Science 2003). The first studies focused on relatively simple systems. Further advances in source brightness have opened up even complex organic systems and solution phase reaction dynamics to atomic inspection. A number of different chemical reactions will be discussed from electrocyclization with conserved stereochemistry (Jean-Ruel et al JCP B 2013), intermolecular electron transfer for organic systems (Gao et al Nature 2013), metal to metal electron transfer (Ishikawa et al, Science 2015), to the recent observation of coherently directed bond formation using the classic I3- system, in a process analogous to a quantum Newton’s cradle (Xian et al Nature Chem 2017). These studies have discovered that these nominally 100+ dimensional problems, representing the number of degrees of freedom in the system, distilled down to atomic projections along a few principle reaction coordinates. The most dramatic example will be shown for the first all atom resolved chemical reaction with sub-Å and 100 fs timescale resolution (Ishakawa, Hayes et al Science 2015) – the fundamental space-time resolution to following the primary processes of chemistry. At this resolution, without any detailed analysis, the key large-amplitude modes can be identified by eye from the molecular movie. This reduction in dimensionality appears to be general, arising from the very strong anharmonicity of the many body potential in the barrier crossing region. We now are beginning to see the underlying physics for the generalized reaction mechanisms that have been empirically discovered over time. The "magic of chemistry" is this enormous reduction in dimensionality in the barrier crossing region that ultimately makes chemical concepts transferrable. How far can this reductionist view be extended with respect to complexity? In this respect, atomically resolved protein functions provide a definitive test of the collective mode coupling model (Miller Acc. Chem. Research 1994) to bridge chemistry to biology, which will be discussed as the driving force for this work.

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Thursday, Sept. 14, 2017

It’s not much of a stretch to tune elastic properties by means of magnetism

Christian Binek, Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska–Lincoln
Christian Binek stretches an elastic band

The space shuttle Challenger accident in 1986 is a tragic reminder that the temperature dependence of elastic properties plays a critical role in the performance of complex machines. At launch day, low ambient temperature, T, caused hardening of rubber O-rings in a solid rocket booster with subsequent catastrophic failure. Since then, also scientific interest in elastic properties has revived in a variety of fields. Those range from studies of abnormal elastic properties in Earth’s lower mantel to the investigation of magnetic field dependence of phonons and include magnetocaloric, elastocaloric, and barocaloric phenomena with modern cooling applications. In this colloquium, I present insights gained from thermodynamics which reveal new aspects on the interdependence of elasticity and magnetism with implications for potential applications. Specifically, I make the case that, in the ergodic regime, the T-dependence of Young’s modulus, E, is determined by magnetic properties. For the large class of materials with paramagnetic or diamagnetic response, simple functional forms of dE/dT vs. T are derived and compared with experimental data and empirical results. A remarkably simple and profound consequence is obtained for superconducting materials. Superconducting materials in the Meissner phase are ideal diamagnets. The thermodynamic consideration implies that constant diamagnetic susceptibility below Tc translates into dE/dT=0. The result applies for single crystalline and ceramic superconductors alike. I conclude with an outlook on tailoring E through design of magnetic and dielectric properties.

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Thursday, Oct. 12, 2017

Nanoscale Devices based on Two-dimensional Materials

Wenjuan Zhu, Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign
Depiction of layered bands with Graphene

Two-dimensional (2D) materials are layered crystals with strong in-plane covalent bonds and weak interlayer van der Waals bonds. These materials have many unique chemical, mechanical, optical and electrical properties, which not only provide a platform to investigate fundamental physical phenomena but also may provide solutions to some of today’s most pressing technological challenges. In this talk, I will present our work on understanding the electrical properties of graphene, transition metal dichalcogenides, black phosphorus, group IV chalcogenides, and their heterostructures.1-3 I will also present our work on the nano-scale electronic devices (logic devices and radio frequency devices) and photonic devices (plasmonic devices and photo-detectors) based on these 2D materials.4,5

  1. Zhu et al., Nat. Commun., 5, (2014).
  2. Zhu et al., Nano Lett., 12, 3431-3436, (2012).
  3. Lu et al., 2D Materials, 3, (2016).
  4. Freitag et al., Nat. Commun., 4, (2013).
  5. Yan et al., Nat. Photonics, 7, 394-399, (2013).

Bio: Wenjuan Zhu received her Ph.D. degree in the Department of Electrical Engineering at Yale University in 2003. After graduation, she joined IBM Semiconductor Research and Development Center, where she made key contributions to the 65nm and 32nm CMOS technology nodes. In 2008, she joined the IBM T. J. Watson Research Center and worked on developing 2D materials based nanoelectronics and nano-photonics. In 2014, she joined the faculty in the Department of Electrical and Computer Engineering at University of Illinois at Urbana-Champaign as an assistant professor. Prof. Zhu’s research focuses on 2D materials, including graphene and layered transition metal dichalcogenides, and nanoscale devices. She received the NSF CAREER award in 2017, IBM Research's Pat Goldberg Memorial Best Paper Award in 2013, Outstanding Technical Achievement Award in IBM in 2008, more than ten Invention Achievement Award in IBM, Henry Prentiss Becton Graduate Prize for exceptional achievement in research in Engineering and Applied Science at Yale University in 2003.

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Thursday, Nov. 9, 2017

Resolving Power of Jets

Sevil Salur, Department of Physics and Astronomy, Rutgers University
Depiction of particle collision from CMS

The phase diagram of QCD matter has been studied with ultra-relativistic heavy ion collisions. The Large Hadron Collider (LHC) at have been delivering heavy ion collisions (Pb+Pb) along with reference data of pp and p+Pb collisions since 2010. With its unprecedented reach in energy, LHC explores new regions of the phase diagram of Quantum Chromo Dynamics (QCD) that can resolve fundamental questions regarding quark confinement, such as "What determines the key features of QCD?" and "How does the hot QCD respond to jet energy loss?". At Rutgers, we have performed multiple, complementary jet measurements at LHC using the CMS detector to answer these questions. This talk will present an overview of our experimental results that reveal detailed properties of the hot QCD matter.

Bio: Dr. Sevil Salur is an associate professor in the Department of Physics and Astronomy at Rutgers. Since earning her Ph.D. at Yale University, she has studied experimental high-energy nuclear physics and investigated the properties of strongly interacting, very hot and dense matter produced in the Large Hadron Collider (LHC) at CERN and the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Long Island, NY. In the course of her experimental work, Salur explores how matter originally formed.

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