O.M. Stewart Colloquium

Every Monday, at 4 PM the department of Physics and Astronomy hosts the O. M. Stewart Colloquium, in rm 120, Physics Bldg. Refreshments are served starting at 3:30 PM in the Physics Library (rm 223, second floor).

Spring 2017
Date Speaker/Title/Abstract
02/27/2017 Prof. David Mandrus - U. Tennessee
Designing Emergent Matter: How Physicists Think about New Materials
Although new materials are the engine that drives much of condensed matter physics, the conceptual process by which new materials are discovered is rarely discussed. In this talk I will try to remedy this situation, and will discuss strategies for finding new materials that challenge our physical understanding. I will also discuss the role of theory in this process, stressing that new materials can be regarded as a playground for testing new theoretical ideas. Several examples will be discussed.
03/20/2017 Prof. Paul J. Kelly - U. Twente, The Netherlands
Turning up the Heat in First Principles Quantum Spin Transport
The spin Hall angle (SHA) is a measure of the efficiency with which a transverse spin current is generated from a charge current by the spin-orbit coupling and disorder in the spin Hall effect (SHE). In a study of the SHE for a Pt|Py (Py=Ni80Fe20) bilayer using a first-principles scattering approach, we find a SHA that increases monotonically with temperature and is proportional to the resistivity for bulk Pt. By decomposing the room temperature SHE and inverse SHE currents into bulk and interface terms, we discover a giant interface SHA that dominates the total inverse SHE current with potentially major consequences for applications [1]. To study bulk Pt, we set up a scattering geometry consisting of two crystalline semi-infinite Pt leads sandwiching a scattering region of length LPt of disordered Pt with atoms displaced from their equilibrium positions by populating phonon modes, as sketched in Fig.1a. For the resistivity and spin-flip diffusion length, this approach has been shown to yield essentially perfect agreement with experiment [2]. We study the SHE by calculating local longitudinal and transverse charge and spin current densities in the scattering region so that both intrinsic and extrinsic contributions are naturally included. To study interface effects, we model a Py|Pt bilayer by matching 9x9 interface unit cells of Py to unit cells of Pt [3] including both lattice and spin disorder in Py. Fig.1b shows the results obtained for bulk Pt at room temperature. This presentation will introduce and review the computational procedures that make these calculations possible [4]. [1] L. Wang, R.J.H. Wesselink, Y. Liu, Z. Yuan, K. Xia and P.J. Kelly, PRL116, 196602 (2016) [2] Y. Liu, Z. Yuan, R.J.H. Wesselink, A.A. Starikov, M. van Schilfgaarde and PJK, PRB91, 220405(R) (2015) [3] Y. Liu, Z. Yuan, R.J.H. Wesselink, A.A. Starikov and P.J. Kelly, PRL113, 207202 (2014) [4] Z. Yuan et al. PRL113, 266603 (2014); PRL109, 267201 (2012); A.A. Starikov et al. PRL105, 236601 (2010)
04/10/2017 Prof. Ian Ferguson - Missouri Science and Technology, Rolla
Are the Dilute Magnetic III-Nitrides a Route to Room Temperature Spintronics?
In this presentation, the current theoretical and experimental status of transition metal and rare earth doped III-Nitrides are discussed and their suitability for room temperature spintronic applications is reviewed.  Reports of room temperature ferromagnetism in these materials are complicated by disparate crystalline quality and phase purity, as well as conflicting theoretical predictions as to the nature of ferromagnetic behavior.  For example, it is still not well understood whether the ferromagnetism derives from an intrinsic material property or from nano-scaled cluster distributions in the system.  In addition, when ferromagnetism is observed it is not clear if it is free carrier mediated as there have only been a few reports of Anomalous Hall Effect and Circular Magnetic Dichroism measurements.  In this work, III-Nitride materials and quantum structures have been grown by metal organic chemical vapor deposition doped with Mn, Fe, Cr and Gd.  The predominant theoretical models and predictions for ferromagnetism in the III-Nitrides are compared with the available literature.  In particular, the correlation of the structural, optical, and magnetic behavior in Dilute Magnetic III-Nitride Semiconductors are analyzed and compared to materials produced by other growth techniques.  A complete understanding of these materials, and ultimately intelligent design of room temperature spintronic devices, will require an exploration of the relationship between the processing techniques, resulting transition metal or rare earth atom configuration, defects, electronic compensation and other physical properties coupled to accurate theory.
04/17/2017 Prof. Min Yun - U. Massachusetts
Dark Side of the Cosmic Star Formation History
The growth in number, mass, and types of galaxies seen across the cosmic time and their stellar mass build-up history are some of the key observables that guides the theory of galaxy formation and evolution and also serve as important constraints on the properties of our Universe itself.  This has been one of the most urgent topics of research in astronomy in recent decades and an important science driver for current and future astronomical facilities such as the Hubble Space Telescope (HST), James Webb Space Telescope (JWST), Herschel Space Observatory, the Atacama Large Millimeter/submillimeter Array (ALMA), and the Large Millimeter Telescope (LMT).  While the pioneering works in this field started with the study of distant galaxies using the HST in the UV and optical light, I will discuss how new infrared and millimeter/submillimeter observations are sheding new light on this classic problem.  I will also highlight some new challenges revealed by recent observations and speculate on prospects for the future research in this area.
04/24/2017 Prof. Deepak Singh, MU
Spring of surprises in artificial honeycomb lattice
Fall 2016
Date Speaker/Title/Abstract
08/29/2016 Dr. Alaska Subedi - Ecole Polytechnique, Paris, France
Theory for light-control of materials properties using mid-infrared pulses
Controlling and enhancing the physical properties of materials is a challenge for both potential applications and basic science research. Recently, low-energy mid-infrared light pulses that couple to the infrared-active phonon modes of a material have been used as a novel way to control the materials in an ultrafast time scales. However, how the light pulses, which impart oscillating forces, can cause persistent and coherent changes to materials properties has not been fully elucidated. In this colloquium, I will present a microscopic theory for such phenomena that is based on symmetry principles, realistic calculations of the energy landscape of the materials as a function of phonon coordinates, and the solution of the equations of motion of the lattice. I will also discuss a recent prediction for ultrafast switching of ferroelectrics using this technique.
09/05/2016 Labor Day Holiday - No Colloquium
09/12/2016 Prof. Tai C. Chiang - U. Illinois - Urbana Champaign
Charge Density Waves in Ultrathin Films
Films as thin as a single molecular layer can exhibit novel properties. For an illustration, this talk focuses on a model system: titanium diselenide (TiSe2). It belongs to a vast family of transitional metal dichalcogenides, many of which show charge density wave (CDW) or superconducting transitions. Bulk TiSe2 exhibits a particularly simple (2x2x2) CDW transition at TC ~205 K, but the nature of the transition has been under debate for decades. A detailed investigation of the electronic structure by angle-resolved photoemission spectroscopy (ARPES) is complicated by the three-dimensional nature of the CDW order. A single layer of TiSe2 has a simpler two-dimensional electronic band structure. Experimentally, it exhibits a (2x2) CDW transition at TC = ~232 K, which is higher than the bulk TC. The question is – why? Our measurements reveal a small absolute band gap at room temperature, which grows wider with decreasing temperature T below TC in accordance with a BCS-like mean-field behavior. The corresponding atomic displacements, determined by synchrotron x-ray diffraction, also follow a BCS-like mean-field behavior. The results taken from the single layer, multilayers, and bulk crystals provide some detailed answers to long standing questions about CDW physics.
09/26/2016 Prof. Roy Maartens - U. Western Cape
Justin Huang Memorial Lecture: Probing the Universe at Radio Wavelengths
The international Square Kilometre Array telescope is moving closer to reality - with the South African pathfinder telescope MeerKAT already nearing completion. In this talk I will review the basic ideas underlying radio astronomy, and describe the MeerKAT array and its extension that will become Phase 1 of the SKA Mid-frequency array. I will point to the range of science cases for the SKA, highlighting the synergy with astronomy at optical and other wavelengths.
10/10/2016 Prof. Mark Tuominen - University of Massachusetts
Quantum Physics in Nature: Electron-Conducting Microbial Nanowires
Nature self-assembles a variety of interesting functionalities and phenomena, some of which utilize quantum phenomena. One example is a special type of bacteria-- Geobacter sulfurreducens--that possesses the ability to make electricity and transport electrons over long distances through protein nanofilaments, pili, which we call “microbial nanowires”. I will discuss an experimental investigation over the last decade during which time the story of this phenomenon has been unfolding. Conductivity of a network of these nanowires is seen to increase with lowering temperature, indicating metallic-like transport similar to that of some synthetic conducting polymers and organic conductors. The conductivity can be modulated, like a transistor, by electrochemical or field-effect gating. It has been proposed that the electron transport though pili protein occurs through the aromatic amino acids forming a conducting chain within pili. X-ray studies of pili show that the aromatic amino acids are packed close enough for pi-pi orbital wavefunction overlap to occur, thereby conferring efficient conductivity to the nanofilament. Recent studies on genetically modified bacteria, with varying amount of aromatic amino acid density, demonstrate that the magnitude of the conductivity can be engineered by a considerable amount. [1] G. Reguera, et al. Nature 435:1098-1101 (2005); [2] N. Malvankar, N. et al. Nature Nano. 6, 573-579 (2011); [3] N. Malvankar, N. S. et al. mBio 6, e00084-00015 (2015); [4] Y. Tan et al. Small 12, 4481 (2016).
10/17/2016 Prof. Alan Tennant - Oak Ridge National Laboratory
Neutron Scattering Investigations of Topological Quantum Materials
10/24/2016 Hui Zhao, University of Kansas-Lawrence
Ultrafast Laser Spectroscopy of Two-Dimensional Materials beyond Graphene
Starting with the discovery of graphene in 2004, the interest in two-dimensional (2D) materials since then has been exponentially growing. Across many disciplines, their exceptional electrical, chemical, thermal, and optical properties have drawn considerable attention that created an entire field within a decade of their discovery. Ultrafast lasers are powerful tools to control and probe transient processes in 2D materials and study their optical properties. This colloquium focuses on recent studies of 2D materials using ultrafast laser spectroscopy. First, transient absorption microscopic measurements on excitonic and spin dynamics in 2D materials will be discussed. Second, all-optical injection of ballistic currents in 2D materials by a coherent control technique will be discussed. Third, I will discuss polarization resolved measurements that reveal spin and valley dynamics in 2D materials. Last, I will introduce study of charge transfer in multilayer heterostructures formed by 2D materials by using layer-selective transient absorption measurements.
11/01/2016 Prof. Asegun Henry - Georgia Tech
Special O.M. Stewart Colloquium - A Correlation Based Perspective on Phonon Transport
Please note that the O,M. Stewart Colloquium this week will be on a Tuesday in the Physics Library.   The conventional view and understanding of phonon transport, both through materials and at interfac-es, is based on what is termed the phonon gas model (PGM). The PGM essentially treats the energy of phonons as analogous to gas particles that scatter from each other, boundaries or other imperfections in the system. This approach then hinges on the idea that every phonon has a well-defined velocity, which in turn is only well-defined if the atomic arrangement is periodic. There are many systems of interest, however, that do not consist of a fully periodic atomic arrangement and as a result the con-ventional view of phonon transport breaks down in many instances. Recently, the Atomistic Simula-tion & Energy (ASE) research group has developed an alternative framework for understanding pho-non transport which is based on correlation rather than scattering. Moreover, the new framework pro-vides individual mode contributions to thermal transport, regardless of their character, and thus treats all modes on an equal footing. The net result of this approach is then a rigorous way of assessing and understanding the role that different phonons play in heat conduction, both in the body of a material and at interfaces. This talk will present the more recently developed correlation based perspective of phonon transport and will show examples of seemingly non-intuitive behavior that is not well ex-plained by conventional theory (e.g., the PGM), but is easily understood from a correlation based per-spective.
11/07/2016 Prof. Anthony Caruso - U. Missouri Kansas City
Counter High Power Microwave: Mechanism and Material Considerations
After nearly five decades of development, directed energy weapons (DEW), in the form of high energy lasers (HEL) and high power microwaves (HPM) have transitioned from art-of-the-possible to vehicle deployable, both domestically and abroad. In an effort to prepare for risks posed by those who have been vocal and capable abroad, the DoD is investigating methods to mitigate the effects of DEW, including electromagnetic wave transmissivity control, electromagnetic wave sense and evasion, and electronics error post-processing technologies. The work presented here focuses on the problem of developing methods that can be used to control the transmissivity of HPM to cavities enclosing electronics over the 500-MHz to 20-GHz range for power densities in excess of 10-kW/m2. While many have attempted to address this problem using metamaterials, the polarization, angle-of-incidence, and bandwidth limitations have hindered progress toward a realizable solution. Instead, as will be presented in this talk/discussion, is a series of potential condensed matter approaches, which use the electric or magnetic field of the HPM itself, to cause a transition from a high-to-low transmissivity state, and classes of enabling materials thereof.
11/14/2016 Prof. Gang Cao - U Colorado, Boulder
Age of Oxides: New Physics and Foundations of Modern Technology
Modern condensed matter physics research has produced novel materials with fundamental properties that underpin a remarkable number of cutting-edge technologies. Moreover, whoever discovers novel materials generally controls the science and technology that flows from them. It is now generally accepted that novel materials are necessary for critical advances in technologies. Transition metal oxides have recently attracted enormous interest within both the basic and applied science communities.  However, for many decades, the overwhelming balance of effort was focused on the 3d-elements and their compounds; the heavier 4d- and 5d-elements (which constitute two thirds of the d-transition elements listed in the Periodic Table) and their compounds have been largely ignored until recently. We review the unusual interplay between the competing interactions present in the 4d- and 5d-transition metal oxides, and how they offer wide-ranging opportunities for the discovery of new physics and, ultimately, new device paradigms.   
11/21/2016 No Colloquium - Thanksgiving break
12/05/2016 Dr. Ken Herwig - Oak Ridge National Laboratory
Prospects for a Second Target Station at the Oak Ridge National Laboratory Spallation Neutron Source
Oak Ridge National Laboratory is home to two Department of Energy neutron facilities that comprise a world-leading center for neutron sciences, providing researchers with cutting-edge neutron scattering capabilities.  The Spallation Neutron Source (SNS) first target station and the High Flux Isotope Reactor (HFIR) currently support 30 neutron scattering instruments available in the general user program.  I will briefly review plans to complete the instrument suite at SNS by building out the five remaining empty beam lines and present the possibilities for a re-optimized cold neutron guide hall at HFIR. However, most of this talk will focus on the exciting prospect of constructing a Second Target Station (STS) at the SNS producing the world’s highest peak brightness beams of cold neutrons.  I will discuss the science context that led to the design characteristics of STS and its complementarity to current neutron sources, give an overview of the target and moderator technology that produce high brightness beams and describe the suite of neutron scattering instruments that form the basis for concept development.  I will conclude with a look ahead at a few science examples illustrating the power of STS and an update of near term activities.
Spring 2016
Date Speaker/Title/Abstract
02/01/2016 Byron Freelon – MIT
X-rays, Neutrons and Electrons: Combining Three Probes to Study Quantum Materials
Quantum materials inspire a large amount of current condensed matter physics research. This is because quantum materials consist of numerous material systems that exhibit a wide range of physical properties from piezoelectricity to flexoelectricity to high-temperature superconductivity. Often these interesting properties involve the intricate coupling of physical parameters such as electrical charge, spin, atomic position and orbital symmetry. This talk will present some ways in which X-rays, neutrons and electrons can provide information on the electronic, magnetic, structural – and even orbital— behavior in novel quantum materials. The most heavily studied quantum materials are the high-temperature superconductors. We will present data taken from a class of iron-based materials, known as iron oxychalcogenides La2O2Fe2OM2 (M = S, Se), in which unconventional superconductivity may occur. X-ray and neutron scattering reveal orbitally selective electron correlation features that can be enhanced or diminished by atomic substitution. In addition, these materials exhibit exotic electronic properties: the new insulators possess local Coulomb correlation and inter-orbital hybridization reminiscent of Mott and Kondo insulators. It is shown that their non-magnetically ordered state can be made more conducting by exchanging sulfur with selenium suggesting that metallicity might be achieved through carrier doping or external pressure. Combining X-ray and neutron data allows one to contemplate whether these iron-based insulators might be tuned into an unconventional, high-temperature superconducting state. All of this will form a departure point for presenting how electron scattering can be applied to iron-based superconductors and other quantum materials. Finally, the current renaissance in electron diffraction and its potential to shape the future of quantum materials research will be discussed
02/03/2016 Lei Fang – Northwestern University
Design, Synthesis and Characterization of Topological Insulators and 2D Materials beyond Graphene
Topological insulators (TIs) are electronic materials that have a bulk band gap like an ordinary insulator, but possess conducting states on their edge or surface (1). These states are protected by spin-orbit coupling and time reversal symmetry. TIs demonstrate abundant novel phenomena, such as the dissipationless electric current, the graphene-like electronic structure, and the spin-momentum locking (2). These remarkable properties have significant impact on energy harvesting, spintronics and quantum computing techniques. In this colloquium, I will firstly overview the electronic structure of TIs and the potential applications. The second part of my talk focuses on TI nanomaterials. I will introduce our catalyst-free physical vapor deposition method that enables the growth of millimeter-long topological insulator Bi2Se3 nanoribbons. Angle dependent quantum oscillation measurements on single nanoribbon show clear evidences for topological surface states (3). The long-length nanoribbons open the possibility for fabricating multiple nanoelectronic devices on single TI nanoribbon. In addition to TI nanomaterials, I will briefly talk about how to rationally design new TIs using the so-called structure motifs engineering method (4). We have successfully discovered a new TI system [PbSe]5[Bi2Se3]3m, m=1,2 and observed superconductivity by tuning doping and structure (5). In the last part of my talk, I will introduce our latest progress on 2D heterostructure [Pb2BiS3][AuTe2]. This material exhibits a wealth of interesting properties such as Dirac-like linear band dispersions (6), extremely large electrical anisotropy, unusually strong spin-orbit coupling (7), weak antilocalization, and atomically thin nanosheets. Moreover, our first-principles calculations show a helical-like spin texture on the Fermi surface. The helical-like spin texture obtains an unusual winding number of the spin vector that may give rise to a nontrivial Berry’s phase (7). References (1) X.-L. Qi and S.-C. Zhang, Phys. Today 63(1), 33 (2010). (2) M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010). (3) L. Fang, et al., Nano Lett. 12, 6164(2012). (4) L. Fang, et al., J. Am. Chem. Soc. 136, 11079 (2014). (5) L. Fang, et al., Phys. Rev. B (R) 90, 020504 (2014). (6) L. Fang, et al., J. Am. Chem. Soc. doi:10.1021/ja5111688 (2015). (7) L. Fang, et al., Submitted to Nature Phys.
02/08/2016 Luyi Yang – Los Alamos National Laboratory
Ultrafast optical probes of spin and valley physics in semiconductors
Optical techniques using polarized light are among the most powerful methods for probing spin dynamics of electrons and holes in semiconductors. The key to optical control is the strong spin-orbit selection rules that govern absorption near the bandgap, which permit photo-generation and detection of specific spin states (and in certain special cases, specific valley states). This talk will describe how we have applied state-of-the-art optical techniques to explore new physics in two different classes of two- dimensional semiconductor systems: i) the dynamics of the “persistent spin helix” of electrons in GaAs quantum wells, and ii) spin and valley dynamics of electrons in atomically thin transition-metal dichalcogenides (e.g. MoS2).    First, we describe a new experimental technique – Doppler spin velocimetry [1,2] – that we developed to study the temporal and spatial dynamics of the “persistent spin helix” [3,4], which is a spin texture that can form in n-GaAs quantum wells. Using this method, which can resolve nanometer-scale displacements of the electron spin polarization on subpicosecond time scales, we found that the spin helix velocity changes sign as a function of wave vector and is zero at the wave vector that yields the largest spin lifetime [2], and the velocity of spin polarization packets becomes equal to the drift velocity of the high-mobility electron gas in the limit of small spin helix amplitude [2].    More recently, we have directly measured the coupled spin and valley dynamics of resident electrons in two-dimensional “Dirac semiconductor” molybdenum disulfide (MoS2)   using optical Kerr-rotation spectroscopy [5]. These measurements revealed very long spin lifetimes of resident electrons exceeding  3ns at 5K (i.e., orders of magnitude longer than the typical exciton lifetimes that have been primarily studies to date). In contrast with conventional III-V or II-VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. This suggests a novel mechanism of electron spin dephasing in monolayer, driven by rapidly-fluctuating internal spin-orbit fields due to fast intervalley scattering [5]. [1] Luyi Yang et al., Phys. Rev. Lett. 106, 247401 (2011). [2] Luyi Yang et al., Nature Physics 8, 153-157 (2012). [3] B. Andrei Bernevig, J. Orenstein, and Shou-Cheng Zhang, Phys. Rev. Lett. 97, 236601 (2006). [4] J.D. Koralek et al., Nature 458, 610-613 (2009). [5] Luyi Yang et al., Nature Physics 11, 830-834 (2015).

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