Bulletin of the American Physical Society
2014 Annual Meeting of the Mid-Atlantic Section of the APS
Volume 59, Number 9
Friday–Sunday, October 3–5, 2014; University Park, Pennsylvania
Session H2: 1D and 2D Materials |
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Chair: ChaoXing Liu, Pennsylvania State University Room: Life Sciences Building 004 |
Sunday, October 5, 2014 9:00AM - 9:36AM |
H2.00001: Majorana fermions in semiconductor nanowires coupled to superconductors Invited Speaker: Sergey Frolov Majorana fermions are real solutions to the Dirac equation, meaning they are their own antiparticles. In the condensed matter context, they are quasiparticles that are equal superpositions of electrons and holes. A practical challenge of today is to generate, isolate and study individual Majorana fermions. Theory tells us that they may arise is topological superconductors characterized by spinless p-wave pairing. A particularly feasible approach to realizing this unconventional superconducting state is in hybrid structures of a conventional superconductor and a semiconductor with spin-orbit coupling. I will discuss our experiments on semiconductor nanowires that explore this approach, and present the signatures of Majorana fermions obtained by low temperature transport measurements. Interest in Majorana fermions is in part fueled by their predicted but not demonstrated non-Abelian property, which is key to applications in topological quantum computing. I will describe how we can study this in semiconductor-superconductor devices. [Preview Abstract] |
Sunday, October 5, 2014 9:36AM - 9:48AM |
H2.00002: Large-Area Si-Doped Graphene: Controllable Synthesis and Enhanced Molecular Sensing Simin Feng, Ruitao Lv, Maria Cristina dos Santos, Claire Antonelli, Kazunori Fujisawa, Ayse Berkdemir, Rodolfo Cruz-Silva, Ana L. Elias, Nestor Perea-Lopez, Florentino Lopez-Urias, Humberto Terrones, Mauricio Terrones Large-area Si-doped Graphene (SiG) sheets have been synthesized for the first time using methoxytrimethylsilane and hexane as precursors in a bubbler-assisted chemical vapor deposition setup. As a proof-of-concept, their application in probing different organic molecules was successfully demonstrated. We noted that significant enhanced molecular sensing was achieved when SiG was used as a probing surface in virtue of their enhanced Raman scattering effect. This unique enhancement of SiG was explained using \textit{ab initio} calculations, in which local distortions caused by the presence of Si atoms increase the interaction of the dye molecules with the doped graphene surface, in addition to the presence of an incomplete valence electron caused by the Si atom. Subsequently, the laser electronic excitation generated in SiG is then transferred to the molecule, and give rise to the strong Raman scattering effect. [Preview Abstract] |
Sunday, October 5, 2014 9:48AM - 10:00AM |
H2.00003: Low-Energy Electron Reflectivity of Two-dimensional Materials Qin Gao, Patrick Mende, Nishtha Srivastava, Michael Widom, Randall Feenstra Based on density functional theory, we develop a self-consistent description of low-energy electron reflectivity spectra of both free-standing thin films and thin films on substrates. Our approach utilizes wavefunctions for a thin multilayer slab together with wavefunctions of bulk substrate, if any [1, 2]. Our results compare well with experimental data for graphene on SiC and on various metallic substrates. From our modelling, we find that the minima of reflectivity arise from states with wavefunctions localized \textit{between}the graphene layers, rather than on the layers as previously suggested [3]. The energies of the reflectivity minima are sensitive to the layer spacing between graphene and substrate; thus our method also provides a way to determine the layer spacing by comparing with the experimental reflectivity curve. A simple method is also proposed for inclusion of inelastic effects (electron absorption) in the computed reflectivity spectra, and is applied to graphene and hexagonal boron nitride (h-BN) on various substrates [4]. \\[4pt] [1] R. M. Feenstra, N. Srivastave, Q. Gao, M. Widom, G. Diaconescu, T. Ohta, G. L. Kellogg, J. T. Robinson and I. V. Vlassiouk, Phys. Rev. B 87, 041406(R) (2013). [2] N. Srivastava, Q. Gao, M. Widom, R. M. Feenstra, S. Nie, K. F. McCarty and I. V. Vlassiouk, Phys. Rev. B 87, 245414 (2013). [3] H. Hibino, H. Kageshima, F. Maeda, M. Nagase, Y. Kobayashi, and H. Yamaguchi, Phys. Rev. B 77, 075413 (2008). [4] Q. Gao, P. C. Mende, M. Widom, and R. M. Feenstra, submitted (2014). [Preview Abstract] |
Sunday, October 5, 2014 10:00AM - 10:12AM |
H2.00004: Transport determination of tunable band gap in bilayer graphene J. Li, J. Todd, K. Watanabe, T. Taniguchi, J. Zhu Bernal stacked bilayer graphene is a unique two-dimensional material with a tunable band gap. A perpendicular electric field can break the inversion symmetry of the two graphene layers and open up a field-dependent band gap $\Delta $ (E) up to 0.25 eV. Although $\Delta $ (E) have been measured by optical spectroscopy [1], transport determination is hindered by stronger disorder in oxide- supported samples [2]. By using high-quality dual hexagonal boron nitride gated samples, we measure the temperature dependence of the charge neutrality point resistance, from which $\Delta $ (E) is determined. We find $\Delta $ (E) to increase approximately linearly with the applied displacement field D and reach $\sim$ 0.2 eV at D$=$1.6V/nm. The transport results are close to previous optical measurements but with much higher accuracy. Comparisons to theory and measurements in oxide-supported samples are made. An electric field tunable clean band gap in high quality bilayer graphene can be potentially useful in near and mid-infrared light detection.\\[4pt] [1] Y. Zhang, et. al., Nature 459, 820-823, (2009).\\[0pt] [2] K. Zou and J. Zhu, PRB 82, 081407 (R) (2010). [Preview Abstract] |
Sunday, October 5, 2014 10:12AM - 10:24AM |
H2.00005: Screening of substrate charged impurities as mechanism of conductance change in graphene gas sensing Sang-Zi Liang, Gugang Chen, Avetik Harutyunyan, Jorge Sofo In graphene gas sensing, the measured conductance change after the sensor is exposed to target molecules has been traditionally attributed to carrier density change due to charge transfer between the sample and the adsorbed molecule. However, this explanation has many inconsistencies when it is applied to graphene on silica substrate. In this talk, we propose and explore an alternative mechanism. When adsorbed, charged functional groups and polar molecules on the surface of graphene may counteract the effect of charged impurities on the substrate. Because scattering of electrons with these charged impurities has been shown to be a limiting factor in graphene conductivity, this leads to significant changes in the transport behavior. A model for the conductivity is established using the random phase approximation dielectric function of graphene and the first-order Born approximation for scattering. The model predicts magnitudes for the charge and dipole moment which has maximal screening effect. The dipole screening is generally weaker than the charge screening although the former becomes more effective with higher gate voltage. With increasing amount of adsorbates, the charge impurities eventually become saturated and additional adsorption always leads to decreasing conductivity. [Preview Abstract] |
Sunday, October 5, 2014 10:24AM - 10:36AM |
H2.00006: Challenges of Room Temperature Scanning Tunneling Microscopy investigation of carbon nanotubes on a HOPG substrate Morewell Gasseller, Jessica Ritchie, Erin McCarthy Highly oriented pyrolytic graphite (HOPG) is a common substrate for STM studies of carbon nanotubes. It is an ideal surface for STM because it is easily cleavable by adhesive tape, resulting in large, atomically flat planes that are relatively inert and electrically conducting. Despite these attractive attributes, the cleavage of HOPG surfaces also generates a variety of artifacts, some of which are elongated structures similar to the carbon nanotubes being investigated. Some even exhibit periodicities that mimic the atomic structures expected in the carbon nanotubes. In our investigation of SWCNT deposited on a graphite substrate, we observed and catalogued many of these commonly known filament --like artifacts. The data presented here serve as a demonstration for how we differentiated SWCNT from filament-like graphite artifacts in STM experiments. [Preview Abstract] |
Sunday, October 5, 2014 10:36AM - 10:48AM |
H2.00007: Quantizing Nonholonomic Systems Oscar E. Fernandez Recent developments in nanomanufacturing have produced molecular ``nanocars'' that roll on (usually) gold surfaces. Macroscopically, these nanocars are just cars, which are classic examples of nonholonomic systems---mechanical systems subject to non-integrable velocity constraints. Data on the energy required to set the nanocars in motion exists, but no theory of ``quantum nonholonomic mechanics'' exists. In this talk I will discuss my recent article developing such a theory for particular classes of nonholonomic systems, and my current work on extending that theory to develop a theoretical model for nanocars' quantum dynamics. [Preview Abstract] |
Sunday, October 5, 2014 10:48AM - 11:00AM |
H2.00008: Adsorption-induced shape transitions in bistable carbon nanotubes Eric M. Mockensturm, Oleg E. Shklyaev, Milton W. Cole, Vincent H. Crespi Large diameter single wall carbon nanotubes are capable of changing geometry in response to the presence of physically adsorbed gas inside. Coupled to a gas reservoir, an initially collapsed tube can expand so that the adsorbent forms concentric shells on the inner part of the tube wall. Using a lattice gas model, we describe the evolution (as a function of gas chemical potential) of the configuration of the nanotube and the absorbed gas shells at zero temperature. The resulting tube shape and the number of the absorbed shells depend on the tube diameter and the species of the absorbed gas. [Preview Abstract] |
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