Bulletin of the American Physical Society
Annual Meeting of the APS Four Corners Section
Volume 62, Number 17
Friday–Saturday, October 20–21, 2017; Fort Collins, CO
Session L2: Condensed Matter and Materials VI |
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Chair: J R Dennison, Utah State University Room: Lory Student Center 376 |
Saturday, October 21, 2017 11:10AM - 11:34AM |
L2.00001: Valleytronics with interlayer excitons in MoSe$_{\mathrm{2}}$-WSe$_{\mathrm{2}}$ heterostructures Invited Speaker: John Schaibley Monolayer transition metal dichalcogenides (MoSe$_{\mathrm{2}}$, WSe$_{\mathrm{2}})$ are direct bandgap semiconductors. The optical properties of these two-dimensional (2D) semiconductors are dominated by excitons that occur at two nonequivalent (K and --K) valleys on the edge of the Brillouin zone. In this presentation, I will discuss the electronic and optical physics of 2D heterostructures that are fabricated by vertically stacking single monolayers of WSe$_{\mathrm{2\thinspace }}$and MoSe$_{\mathrm{2}}$ together. The resulting 2D heterostructure realizes a type-II junction, allowing for the formation of interlayer excitons with the electron in the MoSe$_{\mathrm{2}}$ layer and the hole in the WSe$_{\mathrm{2}}$ layer. I will show that the valley physics of the monolayers is inherited by the interlayer excitons, evidenced by helicity dependent photoluminescence measurements, and report on the dynamics of interlayer exciton relaxation. I will discuss applications of interlayer excitons to valleytronics, which in analogy to spin in spintronic, seeks to utilize valley polarizations for low power information processing. [Preview Abstract] |
Saturday, October 21, 2017 11:34AM - 11:46AM |
L2.00002: Optically Detected Magnetic Resonance; Computational Predictions and Experimental Results Scott Crossen, John Colton Electron spin resonance (ESR) is an important tool in understanding the quantum-mechanical properties of condensed matter. When coupled with a photoluminescence measuring component, it is possible to optically record ESR information contained in the resulting induced light. This unique form of ESR is called optically detected magnetic resonance (ODMR). In this presentation we compare experimental ODMR data with ESR predictions generated from a computational modeling system known as "EasySpin". To investigate the differences between these two methods we will study one spin-system in particular: irradiated 4H silicon carbide. This specimen will serve as the primary means to connect the two very different forms of computational and practical ESR spectroscopy commonly used today. Methods and theory for both methods will be accurately described and resulting spectra will be presented for comparison. Though there will always be some differences, results show that computational ESR predictions match experimental results to the same extent that the underlying Hamiltonian for that particular system is understood. [Preview Abstract] |
Saturday, October 21, 2017 11:46AM - 11:58AM |
L2.00003: Edge Effects on Vortex Nucleation in Superconducting Granular Aluminum Films W. F. Maughan, A. D. DeMann, S. B. Field Type-II superconductors experience an intermediate state in which quantized magnetic flux lines, or vortices, penetrate the sample in the presence of a magnetic field. An applied current nucleates these vortices on one edge of the film, and then drives them to the other edge where they exit the sample. Because this nucleation process occurs at the sample edge, the details of the edge geometry are critical. In order to probe the effects of a tapered edge on vortex nucleation, a granular aluminum-oxide sample with five segments was fabricated. Each of the segments has a tapered edge on one side and shares the same vertical reference edge on the opposing side. In our experiments, the currents required to nucleate vortices at each edge were measured as a function of the applied magnetic field. These critical currents indicate the magnitude of the barrier to nucleation created by the edges. A clear trend between the size of the barrier and the taper length is observed. [Preview Abstract] |
Saturday, October 21, 2017 11:58AM - 12:10PM |
L2.00004: Focusing of High Wavevector Phonons J J Bible, R E Camley In isotropic materials, energy leaving a point source often spreads out uniformly in all directions. In contrast, in materials that are anisotropic the energy can sometimes leave the source in narrow beams, known as caustics. In elastic materials, this is known as phonon focusing and has been studied in the long-wavelength limit both experimentally and theoretically. Surprisingly there have been very few studies of focusing with high wavevector phonons, where the wavelengths are short enough that they see the lattice structure. We show that this effect leads to focusing of high-wavevector phonons even though long wavelength waves are not focused. We use both analytic and numerical methods. The numerical model, a system of coupled atoms in two dimensions, allows us to explore new features. We study the effect of impurities in the system, nonlinear effects, and the decay of the intensity with distance from the source. We find that at short distances the intensity varies as r$^{\mathrm{-n}}$ where n ranges from .56 to .69. This is in contrast to the far field limit which has n $=$ 1. Furthermore, small nonlinear coefficients lead to significant changes in the focusing pattern of the system. [Preview Abstract] |
Saturday, October 21, 2017 12:10PM - 12:22PM |
L2.00005: Photochemistry with Diamond Jonathon Barkl, Anna Zaniewski, Robert Nemanich In this project, we are exploring thin films of diamond on various substrates for photochemistry through electron emission induced by light in the visible spectrum. Diamond is unique as a semiconductor due to its large 5.5 eV band gap, and can have a negative electron affinity, meaning the conduction band edge is at a higher energy than the vacuum. This property allows the electrons emitted through photoemission to be used as an energy ``reservoir'' for energy intensive reduction reactions, such as the reduction of nitrogen gas to ammonia. This project explores the properties of the diamond films, substrates, and experimental setups in order to determine if photoemission and chemistry are possible with diamond in the visible light spectrum, with lower photon energies than previously demonstrated. This will require the lowering of the effective work function, the energy required to excite electrons from the valence band to the conduction band. The first phase of this project will be to recreate previous experimental results achieved using ultraviolet light on diamond films on molybdenum substrates. In the second phase, we will study the physical properties of the diamond films, various substrates, and other properties in order to achieve the necessary low effective work function. [Preview Abstract] |
Saturday, October 21, 2017 12:22PM - 12:34PM |
L2.00006: Charge-Plasma Coupling and the Transition from Dynamic to Static Disorder David H. Dunlap, Tzu-Cheng Wu The low density of injected charges in molecular solids for device applications such as photoconductors and solar cells gives rise to a small plasma frequency. This in turn implies a slow evolution of the potential energy landscape arising from thermal fluctuations in charge density. For a sufficiently dilute system, a dynamic disorder -- to - static disorder transition occurs when the movement of any one charge is faster than the characteristic oscillations of the landscape through which it travels. In this talk we will show how this behavior can be described in the context of polaron transport by including the appropriate dispersion in the Froehlich Hamiltonian. [Preview Abstract] |
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