Department of Physics

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Colloquia & Seminars, Fall 2017

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Giant Spin-Orbit Torque From Bismuth Selenide Thin Films

Speaker: Mahendra D C
Date: Friday, November 17, 2017
Time: 4pm
Room: 205 Currens Hall

Abstract: The spin-orbit torque (SOT) arising from materials with large spin-orbit coupling promises a path for ultra-low power and fast magnetic-based storage and computational devices. We investigated the SOT from magnetron-sputtered BixSe(1-x) thin films in BixSe(1-x)/CoFeB heterostructures by using dc planar Hall and spin-torque ferromagnetic resonance (ST-FMR) methods. Remarkably, the figure of merit known as the spin Hall angle (SHA) was determined to be as large as 18.62 ± 0.13 and 8.67 ± 1.08, using the dc planar Hall and ST-FMR methods, respectively. Moreover, switching of perpendicular CoFeB multilayers using SOT from the BixSe(1-x) has been observed at room temperature (RT) with critical switching current density 4.3 × 105 A/cm2, which is the lowest-ever critical switching current density reported in a bilayer system. The transmission electron micrographs (TEM) show that the sputtered BixSe(1-x) films have a polycrystalline structure, whose grain size decreases with the film thickness. Quantum transport simulations using realistic sp3 tight binding model suggests that the giant SOT in sputtered BixSe(1-x) is due to a quantum confinement effect, whose spin-accumulation enhances with reduced size and dimensionality. The demonstrated giant SHA, ease of growth of the films on a silicon substrate, and successful growth and switching of perpendicular CoFeB multilayers on BixSe(1-x) film provide an avenue for the use of the topological insulator (TI) BixSe(1-x) as a spin-current generator in SOT-based memory and logic devices.

About the speaker: Mahendra D C graduated from WIU with a MS in Physics in Fall 2011. Currently, he is pursuing his Ph. D. in physics at the Department of Physics, University of Minnesota, Minneapolis. His research is focused on the study of materials for magnetic data storage.

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Teaching Physics Without A Textbook: Newspaper Physics

Speaker: Prof. John Hauptman, Iowa State University
Date: Friday, November 3, 2017
Time: 4pm
Room: 205 Currens Hall

Abstract: We will describe the course “Newspaper Physics” that was taught for many years at Iowa State University as a Learning Community combining English and physics into a single 6-credit course. The physics content was driven by whatever appeared in the daily newspapers that contained physics concepts and ideas, and the English involved a lot of writing, in a writers’ workshop style, mostly about physics. We’ll present an action model and themes from a study that followed students in this course for three years.

About the speaker:  Professor John Hauptman's research interests are in the field of particle physics. He leads the 4th detector, a general purpose detector designed for high energy electron-positron collider that can measure and identify all the fundamental Fermions and Bosons of the standard model.

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Shock Wave Compression of Condensed Matter

Speaker: Dr. Jerry W. Forbes
Date: Friday, November 3, 2017
Time: 2pm
Room: 205 Currens Hall

Abstract: Shock wave physics is a way to study condensed matter at high pressures and compressions. The very high pressures can be greater than the pressure at the center of the earth [≈3.6 Mbar] and in stars. Shockwave techniques can only maintain such pressures for a few millionths of a second [µs]. One-dimensional strain experiments will be described as well as the conservation laws that have to be obeyed. A material that can reach compression equilibrium in sub-microseconds such as a fine-grained metal will have a steady shock wave front travel thru the sample. This steady shock profile can easily be measured as a function of time. A review of a couple of different 1-D strain shock wave experiments with gauges will be presented. In addition, proton radiographic pictures of 1-D shock waves in aluminum plates will be shown.

Materials not in mechanical equilibrium within sub-microseconds after being shocked will have shock wave fronts that change shape as they travel through the sample making it much more difficult to characterize these materials. Experiments on these materials will not be discussed in this seminar.

About the speaker: Dr. Forbes has over 50 years experience in conducting research in shock wave and detonation of condensed matter. Dr. Forbes is an internationally known shock and detonation physicist for his work on phase transitions under shock loading, sensitivity and detonation of energetic materials, and ramp wave loading of explosives. He obtained a BS from Western Illinois University in 1963, a MS from University of Maryland in 1967 and a PhD from Washington State University in 1976 with Professor George Duvall as his advisor. He spent thirty-three years from 1963-1996 at the Naval Surface Warfare Center (White Oak and Indian Head laboratories), seven years from 1996-2003 at Lawrence Livermore National Laboratory as leader of the High Pressure Materials group in the Chemistry Directorate, four years (2003-2007) at the Center of Energetic Concepts at the University of Maryland’s mechanical engineering department and has been a senior Physicist at the Center for Energetics Technology Center from 2007. He was the Material Interaction program manager for the Navy’s Chair Heritage electron beam program from 1976-1983. This program evolved into the Nation’s “Star Wars Program” run by the Air Force. Dr. Forbes has published over 100 papers and technical reports. He is well known for his involvement in the American Physical Society’s (APS) Shock Compression of Condensed Matter (SCCM) Topical Group having served as the first Secretary/Treasurer and later as the Chair of the group. He became a Fellow of the APS in 1992 based on his technical work on phase transitions and his service to the SCCM topical group.

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 Optical Properties of Transition Metal Doped Materials for Middle Infrared Laser Applications

Speaker: Dr. No Soung Myoung
Date: Friday, October 27, 2017
Time: 4pm
Room: 205 Currens Hall

Abstract: There have been various suggestions for the fabrication and spectroscopic analysis of novel solid state middle infrared lasers using the transition metals (TMs) doped in II-VI semiconducting materials in the range of 2~6 µm. These mid-IR laser sources are mainly utilized for counter measurement, remote sensing, and chemical sensing. In this talk, we demonstrate the method of fabrication and laser performance of the iron doped II-VI semiconducting polycrystals pumped by a 2.78 µm laser. In addition, the mechanism and lasing of cobalt co-doped Fe:ZnSe and Fe:ZnS pumped by an Alexandrite laser at 755 nm will be discussed.

About the speaker: Dr. No Soung Myoung is a research fellow at the Gwangju institute of Science & Techonolgy (GIST), South Korea. Before joining this institute, he received the M.S. degree in Physics at Western Illinois University (May 2002) and the Ph.D. degree in Physics at the University of Alabama at Birmingham (UAB, 2011). After a short period of post-doctoral study at UAB, he worked as a senior scientist at Samsung Display Co. to build an excimer laser for laser-lift off (LLO) of the flexible display for smartphones. Then, he returned to academia since 2013 through the Advanced Photonics Research Institute at GIST to participate in the development of the middle infrared lasers. His research explores optical properties of transition metal doped materials for middle infrared laser applications using a variety of experimental techniques, including time-resolved spectroscopy and nano~femto-second lasers.

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Topological Quantum Numbers in Condensed Matter Systems- a talk dedicated to the 2016 Nobel Prize in Physics

Speaker: Prof. Carlos Wexler
Date: Friday, September 29, 2017
Time: 4pm
Room: 205 Currens Hall

Abstract: In 1980 Klaus von Klitzing discovered [1] a remarkable quantization of the Hall conductance [2] of a quasi two-dimensional electron gas in a strong magnetic field at low temperatures: the conductance jumps in exact multiples of a fundamental conductance unit (depending only on Planck’s constant and the change of the electron) and is independent on the type of semiconductor used, detailed experimental geometry and the presence of impurities or defects. In fact, this quantization is precise to less than a part per billion (and is currently the standard by which the unit of resistance, the Ohm, is defined!). How is it possible that such high-precision and highly reproducible measurements result from "dirty", and somewhat poorly controlled experimental conditions? The answer came in part from the realization of Thouless and colleagues about the topological nature of the Quantum Hall Effect, the Hall conductance is demonstrated to depend on the topology (a global property), and is thus robust against small defects or imperfections [3]. Another "application" of topology in condensed matter systems came earlier. In early 1970s there was considerable controversy on whether any long-range order could exist in 2D systems due to large-scale fluctuations. Experiments, however, showed clear evidence of low-temperature superfluidity and superconductivity. In 1972 Kosterlitz and Thouless [4] proposed a new type of phase transition based on the emergence, at the critical temperature, of free quantized vortices and anti-vortices. Vortices are a kind of topological defect where the fluid circulates around a singularity or core; they are topological as they correspond to the behavior of the fluid all the way around them! We call these vortices “quantized” because their “strength" can only come in integer multiples of a quantum of circulation (superfluids) or a quantum of flux (superconductors), which depend only on fundamental constants. This makes these objects remarkably robust, as they cannot easily dissipate by smoothly loosing strength. The Kosterlitz-Thouless vortex unbinding transition is able to explain in detail the emergence of two-dimensional superfluidity and superconductivity [5].

Notes:

  1. Von Klitzing was awarded the Nobel in 1985; a second Nobel was awarded in 1998 to H. Stormer, D. Tsui and R. Laughlin for the discovery and explanation of the fractional QHE.
  2. Ratio of longitudinal current to transverse induced voltage.
  3. Thouless and Haldane shared 50% of the the 2016 Nobel Prize for their discoveries (I am not discussing Haldane´s work in this talk). It is remarkable that the QHE has resulted in 3 Nobel Prizes in Physics (1985, 1998, 2016)!
  4. The transition was discovered independently by the late Vadim L. Berezinskii and is often referred to as the Berezinskii-Kosterlitz-Thouless transition. 5. Thouless and Kosterlitz shared 50% of the 2016 Nobel Prize for this achievement. Berezinskii died in 1980; Nobel Prizes are not given posthumously

About the speaker: Professor Carlos Wexler is a Professor of physics and the President of the Graduate Faculty Senate at the University of Missouri, Columbia. His research interests are in theoretical modeling and computer simulations of “nano-sponges”—materials with pores in the nanometer scale—that are capable of storing hydrogen and natural gas, phases and phase transitions observed in numerous quasi-two dimensional systems such as two-dimensional electron systems (Quantum Hall Effects), spins and spin chains (“Extended Universality”), and organic films deposited on a substrate.

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Post-Detonation Nuclear Forensics Using Resonance Ionization Mass Spectrometry

Speaker: Drake Brewster
Date: Friday, September 8, 2017
Time: 4pm
Room: 205 Currens Hall

Abstract: The Laser Ionization of Neutrals (LION) Resonance Ionization Mass Spectroscopy (RIMS) instrument at Lawrence Livermore National Laboratory (LLNL) successfully quantified Uranium isotope ratios in nuclear fallout debris. RIMS is a technology developed as early as the nineteen-eighties. Since its development, technological advancements enabled instruments such as LION to deliver isotope ratios within one-percent uncertainty, with a high useful-yield, and free of isobaric interferences.

About the speaker:  Drake graduated Magna Cum Laude from Western in 2009 with a Bachelors of Science in Physics; minoring in Mathematics and Military Science. Growing up in Jacksonville, Florida, he attended Fletcher High School. Having family in the local Macomb area, Drake applied for a ROTC Scholarship at Western. He was selected as four-year federal scholarship recipient. He was also a member of the Centennial Honors College and served for three years as a First-Year-Experience Mentor.

Upon graduation, Drake received an active duty commission as a 2nd Lieutenant in the Armor Branch. His service included assignments as an infantry platoon leader, battalion current operations officer, and battalion intelligence officer. Drake deployed in support of Operation Enduring Freedom in 2013 to Arghandab, Afghanistan.

He is currently finishing a Masters of Science in Physics at the Naval Postgraduate School in Monterey, California. His thesis research is funded by the Defense Threat Reduction Agency. The research is conducted in cooperation with Lawrence Livermore National Laboratory. His next assignment will be as an instructor at the United States Military Academy at West Point in the Physics and Nuclear Engineering Department.

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