2017-2018 Colloquia Abstracts

Return to the 2017-2018 Colloquia Schedule

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.

Return to the 2017-2018 Colloquia Schedule

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.

Return to the 2017-2018 Colloquia Schedule

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.

Return to the 2017-2018 Colloquia Schedule

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.

Return to the 2017-2018 Colloquia Schedule

Thursday, Feb. 8, 2018

Viewing the Eclipse from 109 Thousand Feet and Road to Get There

Michael Sibbernsen, Department of Physics and Astronomy, University of Nebraska–Lincoln and CEO of Branched Oak Observatory
Kendra Sibbernsen, Metropolitan Community College
Researchers launch a high altitude balloon

The NASA Nebraska High Altitude Ballooning Program, led by Michael and Kendra Sibbernsen, was one of 55 teams across the nation to fly weather balloons within the August 21st Total Solar Eclipse and stream LIVE video to the internet. On this day, the Nebraska team launched three high altitude balloons to take scientific measurements and take photos and video of the Moon’s shadow falling on the Earth. These balloons included the NASA common video streaming payload as well as experiments from Metropolitan Community College and students from Omaha Public Schools. The launches took place at the “Gem Over the Prairie” event at the Stuhr Museum in Grand Island, and had an audience of nearly 7000. The history of this project, eclipse stories, amazing images and video, and an examination of the data from the historic flight will be shared at this program.

Shadow of the eclipse across a NASA-branded dish antenna

Bio: Michael Sibbernsen is a Lecturer of Astronomy here at the University of Nebraska- Lincoln, and the CEO of Branched Oak Observatory. Before working here at the university, Michael was the Science & Technology Coordinator for the Strategic Air Command & Aerospace Museum, and before that, the Outreach Coordinator for the NASA Nebraska Space Grant. Michael has traveled throughout the state delivering hundreds of hands-on science and astronomy demonstrations and workshops for NASA and the Nebraska Department of Education. Earlier this month, the Nebraska Association of Teachers of Science awarded Michael their "Catalyst Award," the organization's highest honor in recognition of significant contributions to science and science education in the state of Nebraska.

Dr. Kendra Sibbernsen currently teaches physics and astronomy at Metropolitan Community College where she is the lead online faculty member for natural science and the head of the undergraduate research group. She has taught college science classes for 20 years. Her research focuses on the fields of high altitude ballooning, cosmic ray studies, and astronomy education research. Kendra is also the CFO for Branched Oak Observatory, a non-profit astronomical park north of Lincoln, NE.

Kendra and Michael, are co-leads for the NASA Nebraska High Altitude Ballooning program launching scientific payloads created by area high school and college students. They also train new teams throughout the state to help create their own ballooning programs.

Return to the 2017-2018 Colloquia Schedule

Thursday, Feb. 22, 2018

Spin Transport in Semiconductors

Paul Crowell, School of Physics and Astronomy, University of Minnesota
Paul Crowell

Semiconductors provide a uniquely hospitable environment for electron spins. Mechanisms for relaxation are weak, allowing spins to survive on time scales (nanoseconds) much longer than those typical of charge transport. This makes semiconductors an ideal host, and there are reasonable prospects for controlling spin transport with “knobs” such as the spin-orbit and hyperfine interactions. Given these possibilities, it is a frustrating fact that an external source of spin-polarized electrons is required to exploit these advantages. I will review the progress over the last decade in developing a technology based on the integration of nature’s best source of spin-polarized electrons (ferromagnetic metals) with semiconductors. It is now possible to inject a significant spin-polarized current into semiconductors from a ferromagnetic source and detect it several microns away. I will provide an overview of the materials physics of these devices, along with a discussion of some of the physics they have been able to address.

Bio: Paul Crowell received his PhD in low-temperature physics from Cornell University in 1994 and was a postdoctoral associate at the CNRS, Grenoble and the University of California at Santa Barbara before joining the faculty of the University of Minnesota in 1997. He is currently a Professor of Physics. Professor Crowell’s research focuses on spin dynamics in ferromagnets on sub-nanosecond time scales and spin transport in hybrid ferromagnet-semiconductor and ferromagnet/normal metal systems. He has been a Sloan Research Fellow, a McKnight Land Grant Professor, and a McKnight Presidential Fellow. Professor Crowell has served as General Chair of the Joint MMM/Intermag Conference (2013) and as Chair of the APS Topical Group on Magnetism and Magnetic Materials (2012-2013). He is a fellow of the American Physical Society.

Return to the 2017-2018 Colloquia Schedule

Thursday, Apr. 12, 2018

The Power of Strong Spin-Orbit Interactions: Challenges and Opportunities in Iridates

Gang Cao, Department of Physics, University of Colorado Boulder
Gang Cao

Effects of spin-orbit interactions in condensed matter are an important and rapidly evolving topic. Strong competition between spin-orbit, on-site Coulomb and crystalline electric field interactions in iridates drives exotic quantum states that are unique to this group of materials. This colloquium offers a brief review of current experimental studies of iridates [1] and emphasize discrepancies between experimental confirmation and theoretical proposals that address superconducting, topological and quantum spin liquid phases. It then reports our most recent study on electrical-current controlled behavior in iridates [2]. Electrical control of structural and physical properties is a long-sought, but elusive goal of contemporary science and technology. This work demonstrates that a combination of strong spin-orbit interactions and a canted antiferromagnetic Mott state is sufficient to attain that goal and points the way to novel possibilities for functional materials and devices [2].

Bio: Gang Cao obtained his PhD from Temple University under direction of Prof. Jack E. Crow in 1993. Dr. Cao subsequently worked at the National High Magnetic Field Laboratory from 1993 to 2002, and then University of Kentucky from 2002 to 2016 before he joined the faculty of the University of Colorado at Boulder. Dr. Cao is Fellow of the American Physical Society. His research encompasses a methodical search for novel quantum materials in the single-crystal form, and a systematic effort to elucidate the underlying physics of these materials. Visit his homepage for details: https://www.colorado.edu/lab/cao/

1. "The Challenge of Spin-Orbit-Tuned Ground-States in the Iridates: A Key Issues Review", Gang Cao and P. Schlottmann, Reports on Progress in Physics 81 042502 (2018); https://doi.org/10.1088/1361-6633/aaa979
2. "Electrical Control of Structural and Physical Properties via Spin-Orbit Interactions in Sr2IrO4", G. Cao, J. Terzic, H. D. Zhao, H. Zheng, Peter Riseborough, L. E. DeLong, Phys. Rev. Lett. 120, 017201 (2018); https://doi.org/10.1103/PhysRevLett.120.017201; Editor’s Suggestion

Return to the 2017-2018 Colloquia Schedule